Org has two ways of archiving tasks -- mark them as archived in the current Org file or move them to an archive file. To keep an Org file from getting too cluttered with archive, most people will probably opt for separate archive files.
Could (should?) BeOrg support archive files? I would guess Org operations would have to expand to include '.org_archive' files, but, since they're Org files as well, little else would need doing (CMIIAW). The question is would this unnecessarily blow up memory on the iPhone?
This is something I've considered over the years, and not come to any conclusion. It would definitely be great if beorg did support proper archiving - however as you say there could be memory issues as archive files can be pretty large. Although looking at my own archive files the largest archive file is only x2 my largest org file - and iOS devices do have quite a lot of memory these days.
It would be interesting to here from users as to how many, like myself, spend most of their day with Emacs a few keypresses away and use beorg very much so as a secondary interface to their org files - or whether they only use beorg or use beorg much more than Emacs. For that second set of users archiving in beorg would be incredibly useful.
Actually, I just thought that I didn't ask the question -- does beorg support same file archiving?
Not yet, but this is something which is much easier to implement so more likely to happen in the near future.
PS: Beorg becomes very useful on (extended) trips when the laptop is a lot of extra weight.