I've been curious for a while about this feature. Anyone who uses AE on a regular basis is quite familiar with the composition trimming hot keys using alt + [ and alt + ]. When trimming the front-end of a comp using the hot key, the program will trim the comp to the exact location of the time indicator. However, when you trim the back-end of the comp, the trimming will actually take place one frame later than the location of the time indicator.
Ex. If I set my time indicator to the 10th frame in my comp, then trim the back-end using alt + ], the comp will be trimmed at the 11th frame instead of the 10th.
This is not necessarily an issue I'm dealing with, it's more of a general curiousity of why that is the default setting - and if there's sound logic to it that will improve my AE workflow overall.
"then trim the back-end using alt + ], the comp will be trimmed at the 11th frame instead of the 10th."
Actually, Ae trims your footage at the 10th frame but show you that the trimmed footage will be visible until the next frame. So, if you trim at frame 10 and proceed to frame 11 your footage will disappear, which means that the trim was done right, but the footage looked like as it was trimmed at the 11th frame, and this is how Ae is trying to show you visually that your footage will be visible the whole duration of frame 10
The current time indicator is always at the start of a frame. You are not cutting a layer, you are trimming it to the end of the last frame you are looking at. This makes a lot of sense when you were doing animation or making composites.
The cut tool in an NLE like Premiere Pro always makes the cut at the start of the current frame. If you delete after the cut you will not see anything, but the next clip will start at the CTI. This makes perfect sense for an editing system.
AE is not hey video editing app. Never has been, probably never will be. Animators and visual effects artists always want to see the last frame they're working on. If you want to perform edits you have to adapt.