Picking up on what CapsuleCorp
said, I think asking to take pictures with a cosplayer is simply indulging the play side more than the cos side. When a friend and I cosplayed as Harry & Draco most people who took pictures wanted to be in them - I figured it's because of the whole "Look who I met!" aspect and since the costume were not that detailed or fancy it didn't really matter. So it's more an appreciation of the character than the costume, which is perfectly fine! We all love to see our favourite characters come to life and even interact with us and as cosplayers we all love to make people believe in us being the characters (to a certain extent of course, but that's a different topic, haha).
Though if someone is simply impressed by the costume and less in fangirl-squee-mode (don't mean that in a negative way, just an example), I'd rather advise them to take a shot without themselves in the frame. Having a regular person/costume from a different series in the same shot can be distracting if you want to capture how gorgeous the costume is.
And at the end of the day there's no reason not to do both if the cosplayer has time! A quick fangirl shot and a nice costume shot