One of the things you'll find if you start to do technical, mixed gas diving, is that you will dive fairly strict plans.
Descend from surface to target 2 minutes
Dive at 200ft for 30 minutes
ascend to 150ft 2 minutes
dive at 150ft for 15 minutes
ascend to 70ft 3 minutes
switch to decompression mixture
stop at 70ft for 15 minutes
ascend to 20ft
stop at 20ft for 20 minutes
ascend to surface
While this is a bogus plan, it's similar to what you'd do for real. The trick in technical diving is to "hit your marks". You are not recalculating things on the fly. Your decompression plan requires that you stick to your plan. When the mess hits the fan, you will follow on of your contingency plans.
The tanks you carry, both in number and mixture in them, are specific to the plan you have laid out, and deviations from that plan could mean you no longer are carrying the mixtures or tanks you need to exit the water safely.
Because you are following a plan that is layed out days or weeks in advance, a dive computer than can recalculate on the fly is typically unnecessary. That said, they can be helpful for hugely varying profiles like we see in some caves. Caves are multiprofile by default. If you are diving to a wreck, you are going to the bottom, and generally doing a fairly square profile.
When I looked at some of my cave profiles, it was amazing to see the variance of depth. Often traveling up and down more than 50ft at a time for 1 - 1.5 hours. Trying to calculate that as a square profile is pretty hard.
Hopefully this information helps.
