If you look at the bike fits of many professional road racers, most of them are well back of the BB. Generally speaking, the only guys who have to worry about being within the 5cm zone are guys who have a track background, and tend to be sprinters. A la Mark Cavendish.

The difference between a road bike and a full suspension mountain bike should be obvious: the suspension. The suspension on a mountain bike sags; this results in the actual STA changing, and getting slacker, the steeper the gradient and/or with more sag.

So, a reasonable increase in STA (on full suspension bikes) makes sense, when accounting for the amount of travel the bike has AND the sag numbers typically used on that type of bike. An FS XC bike run at 20% should not have the same STA as a 160mm run at 35%.

 

To be fair, Specialized makes a setback dropper post. Just sayin.

 

Bike fits have been gradually changing over the years. Even on the road.

If you look at footage from the 90s you will see that a lot of the road racers sat quite far back on their bikes when compared todays riders. I am not sure why fit has changed, I think gearing might be a factor. When people climbed at 60-70rpm they sat way back on their saddles, climbing cadence is modern cycling is quite a bit higher.

I am not sure if the bikes have changed but how riders on sitting on them has changed. It looks like seats have come up, the nose of the saddle has come down and riders have moved forward.

1999:

2018:

 

I had a custom hardtail frame built in 1994 which featured a 76° STA, this was considered radical at the time. I employed a pillar style seatpost (no setback) and I loved how it climbed. Have been a fan of steep STAs ever since. IMO one of the benefits of a steep STA / pillar post is it makes getting up out of the saddle to stand & climb easier. By comparison, I found that moving into a standing position on a bike with a slack STA combined with a layback post was like trying to pull myself up out of a La-Z-Boy recliner.

Generally speaking, one difference between off-road cycling and road cycling is the grades ridden. While there are certainly some crazy steep roads out there, these are fairly unusual. Departments of Transportation spec maximum grades on modern roads. Meanwhile insane grades are commonplace in mountain biking. The forward saddle position of a steep STA combined with a pillar style post aids climbing when the going gets crazy steep. Less of a concern on road bikes.
=sParty

 

I had a custom hardtail frame built in 1994 which featured a 76° STA, this was considered radical at the time. I employed a pillar style seatpost (no setback) and I loved how it climbed. Have been a fan of steep STAs ever since. IMO one of the benefits of a steep STA / pillar post is it makes getting up out of the saddle to stand & climb easier. By comparison, I found that moving into a standing position on a bike with a slack STA combined with a layback post was like trying to pull myself up out of a La-Z-Boy recliner.
=sParty

Yeah, but you're short. Really short.

 

9.8 offers a set back seat clamp.

 

Seat post angle doesn't change on a full suspension bike unless you went back in time to a urt bike. The seat tube is welded to the bb the angle cannot change. I do realize it changes relative to the ground but if you use this argument then you should own a different bike if you lived in the desert or mountains.
Proper bike fit has to encompass your physiology to get the most power and still feel comfortable. Moving the saddle forward and backwards changes the relationship for your knees, ankles, feet.
I would prefer a slacker ST. I have my saddle all the way back on my new bikes. If I ride a steep st my knees start to ache. I find a steep st also is harder to out of the saddle sprint on. You have to be more aware of where the seat is.

 

Seatpost angle IS measured relative to the ground.
But there's static and dynamic which is dependant on sag.

My seats are slammed as far forward as I can get them.

 

I swapped seatposts on my gravel bike without realizing the change was for a zero setback post from one that had some setback. It took some tweaking to get the fore/aft adjusted to my liking.

 

When talking about saddle setback it's worth considering the affect it can have on your pedalling and muscle recruitment. When you see people complaining of cramping in the same muscle groups every ride for example that's something that could be addressed by position. eg:

"When you're at your limit, and are fatiguing heavily, take note of where the fatigue is spread through your legs. If your quads and/or calves are burning, you likely need more seat setback to engage the hamstring complex. If you suffer from hamstring fatigue the seat needs to come forwards. If the load is spread well across all major muscle bellies, you are bang on target. This assumes the rest of the position is good and no abnormal technique is occurring of course. The rider should be able to get off the bike and walk up stairs without too much difficulty - a common problem for cyclists with "dead" legs is to struggle up the stairs after a ride - generally this means too much quadricep recruitment! You should be tired, but not completely incapacitated by a certain muscle group.

Remember that as the seat setback is adjusted, so too does the seat height. Move the seat 10mm back and you will likely need to drop the saddle around 3mm to account for this effect. After any major setback change, saddle height should be re-assessed and fine-tuned."

https://neillsbikefit.com.au/?page_id=364

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I'm in agreement with what some stated above, the effect of saddle setback is very much rider dependent. I'm one who is not majorly impacted by either configuration though I prefer set back (and generally stretched cockpit setup). Though I've ridden rental MTB's and demo's with straight posts on occasion with no issues, all six of the bikes in my stable have setback posts.

I did a Guru fit a few years back and got extra time to play with all sorts of variables and re-tested high wattage output to see where I fell off of "optimal". I messed with higher bar height, shorter "stem", 0 setback vs up to ~25mm back, etc. and it didn't have much of an impact for short bursts at least.

On all day sorts of rides, if my bike setup is not dialed in it can lead to transient or longer lasting pain mostly in the legs (quads or hamstrings) or maybe may back.

Never have cramped up myself. Often wonder if the folks I've encountered who ended up with cramps bad enough form them to dismount due to pain were not hydrating enough or possibly salt depleted.