In what conditions do you want lock-out in the forks?

For me? Never. I'd rather use low-speed compression damping to make the fork not dive/bob excessively, but even if there is a little movement that does not inherently mean it's affecting my ability to accelerate forward to any measurable extent. Some people base this on their visual-perception, in other words if they "see" some bobbing they assume it has to be causing some huge performance loss, and that is not necessarily true. I ride on trails that are pretty rocky and bumpy, and never on roads, although my bikes have advanced enough shocks and suspension systems that on roads it's not going to be too bad if I decide to "sprint" out of the saddle. The other issue concerning the above information is that sometimes roadies get their first suspension fork or bike and start freaking out because its not "rock-solid".

On the other hand, some suspension designs are poor and require an excessive amount of compression damping to not bob, and a lockout can make them accelerate better. Some of these designs are quite popular still because they are inexpensive or for other reasons, but on the other hand when you "lock out" you lose the ability of the suspension to increase your traction when going up nasty loose climbs, so the whole question of "does it climb better locked out" really depends on the surface, the bike's suspension in the first place, and so on. For many of the climbs that I do, a lock-out would be very detrimental. Having an 29er kind of makes up for the loss of traction (such as on my hardtail), but then having a 29er with full suspension would again give me a higher amount of traction and better ability to climb.

A lot of times riders are looking for some sort of magic-edge for them to be able to ride with their faster buddies. They think things such as lockouts and bikes that are a few lbs lighter are going to make some sort of measurable difference, and 95% of the time they do not. Getting stronger makes the difference, as well as riding more often. There's also the whole problem of bike-shops and companies trying to push "race-bikes" and gear, as if to be "serious" about mountain biking and to be fast you need to have a race-bike.