Yeah i have noticed that aswell.
I do it myself haha
I think by hard tagging your character in, you are offering your OP a free combo and your moral sort of goes out of the window. By that point in the match you know your OP has the upper hand and you just feel defeated by offering up your second character on a silver platter

.....
OR It either:
1) People are used to losing 60%+ health (during a random hard tag) in UMVC3 and it's just hard wired into their psyche.
2) They want to play the hero and pull off some epic comeback
I haven't played this game in yonks! I should chuck it in

If you (or anyone) see me in a lobby just join.. i won't be taking that seriously anyhow