Well, maybe I'm on the wrong track here, but don't most typical non-belted rifle calibers headspace on shoulder? If so, than having the correct trim length from shoulder to case mouth is what's important, and the rest isn't as important. (as long, of course, as they chamber)
Proper trim length gives you three things:
1) ability to chamber the round
2) safeguard against pinching the case mouth and exponentially rising pressure
3) consistency for the purpose of accuracy/repeatability
Using this trimmer that indexes off the shoulder
should effectively guarantee you both #2 and #3, but if you have brass where the length from base to shoulder varies quite a bit, no other style of trimmer is going to keep you from #2 or perhaps #1. That's a whole separate issue that needs to be remedied by something other than trimming.
To put it another way--if your OAL is the same between two pieces of brass... but your base to shoulder length varies by a large margin
you are going to have problems if that case mouth ends up sneaking out of the chamber.
With that line of thinking, it's only the price that oughta be keeping you from buying this trimmer!
(I think? Good discussion.)