I'm not entirely sure on what the shelter reservation is for, to be honest! I'm assuming its for when you don't have space but spot a pokemon/egg you REALLY want. Either that, or it's for when there's two pokemon on the same screen that you want, but adopting reloads the screen? Personally, I haven't seen a need for it, I like just going in and picking up an egg I haven't dex'd yet, but I can see for people hunting it would be nice if they could reserve a selection of eggs (TBH I haven't even breached shiny hunting yet I have no idea how people get that many eggs xD) So, I wouldn't mind it? But maybe put it off for a bit.