That is normal.
A maximum load on the cpu has to draw power from the battery because the AC adapter isn't big enough to handle the load.
This has been the case since the very first Intel based MBP in 2006. That machine had a removable battery and dropped to half speed if it was run without the battery installed.
Thanks Retired Engineer, but I don't think it's normal. Well, maybe it is "normal" (or by definition: common) in the 2011 MBP models, but it doesn't means that there is a problem with its design or hardware.
I have an "old" 15" MBP, which I bought in 2007, and since then it never happened to me that the computer is draining or unable to charge the battery while heavy processor intensive processes. I used the exact same softwares and processes in my new 2011 17" MBP, and then is when I detected the problem.
I read on some forums that this is a common issue with 2011 models. I don't know if Apple has an official answer about this or if there is a solution or workaround. Maybe they could simply shipped this machines with a more powerful AC adapter, able to deliver more than 85W, which seems to be not enough.
I will make further investigation about this, because I think it is a serious problem when it is presented in a computer labeled as "Pro", which I understand is designed to be under heavy usage.
anything new about the topic?
I've got the same problem with my MacBook Pro 17 late 2011 and I think it's going to lower the span of battery life (draining some percent under heavy load then refilling the few percent and so on...). I've got the problem with the power supply from my LED Cinema display too (maybe the maximum power it can deliver to a MacBook is limited to 85W).
Praetoria: No, nothing has changed. What you and Txiquim and many others have observed is still the as-designed behavior of all MBPs when they are pushed to their limits. With each new generation of ever-more-powerful and power-hungry CPUs and GPUS, more and more users exceed their AC adapters' output capacity when working their machines hard. As long as Apple continues not to offer a higher-output AC adapter -- which I suspect is because the MBP's case design wouldn't allow the effective dissipation of even more heat -- this drain on the battery under heavy load will be the price one has to pay for using a MBP as a desktop-computer replacement.
I agree with eew, a laptop computer should not replace a desktop, just because it has the processing capability does not mean that it should be pushed to the limit.
If one is afraid of damaging the battery with small discharges then recharging, follow the Apple guide for battery maintenance: http://www.apple.com/batteries/notebooks.html