No, if you have the proper sag you have the proper spring rate, the rest should be done with damping. If the fork blows through it's travel (as my marzocchis do) it's because it has weak low-speed compression. I up the oil levels so they won't bottom out, and with the right amount of sag they run correctly over choppy stuff, but they dive off of everything, and if you add a bunch of "compression", it makes the fork crappy overall. This is basically because marzocchi has never really made anything better than the cartridge that came in the first generation of bombers...until 2010 with the new "Evo" cart. The "Evo" cart isn't even all that advanced, definitely not up to the level of the other manufacturers, but it is a huge step finally and combined with other marzocchi features it makes for something that actually works and can have more low-speed compression without turning the high-speed into crap.
If you are used to riding fox forks where you can't adjust the bottom-out (or can't adjust it without servicing the fork and adjusting the hydraulic anti-bottoming cone), then yes, you might think that you have to get a "REALLY STIFF" spring if you are bottoming your fork or it's diving, but that's kind of the whole reason we have a damper in the fork and not just a spring, so we can control these things. Marzocchi's attempts have been very half-ass over the years. I love my 06 66 chassi and it's a decent fork, but my 888 RC3 WC just doesn't cut it, compared to my avalanche rear shocks it's pretty crappy. I'd love to have a front DH fork that is of the same caliber as my rear shock. I'll probably save up for the Avalanche fork-cartridge that will be comming out for the 888. Due to how half-assed the marzocchi damping system has been for so long, I'll probably skip the Evo, even if it is that much better. They should have fixed that 5 years ago.
You may be right that on some products you'd have to get a higher rate spring to solve some of these issues, but it would run like crap and make all sorts of other issues. Bottom line is the crappy damper design.