I cannot tell you the number of times I've needed to use a synth or pop a glimmer buff while in battle. I'd have no problem finding a quiet corner for a second and loading the menu, but the UI sometimes takes so long to load that it's counter-productive to my fireteam to bother. If I could activate those items from the app, the same way I can do a quick armor or weapon swap, it would be extremely helpful. I don't know if you can turn in bounties from the app, but considering you can now do it from the menu it would be convenient, if not all that necessary. I don't however think we should be able to purchase things from vendors through the app. Especially from Xur. Unless we're at some point working openly with The Nine.