That's exactly what makes it team-centric though, you need the whole team to win, rather than an individual hyper carry. (Technically it's still possible to carry a bit in HotS if your weakest link isn't absolutely abysmal, but that depends on the comps/situation/map, and definitely to a way lesser extent than in the item MOBAs in any case.)
But yes, sometimes you get people having bad games. I've managed to talk tilting players back into line on quite a few occasions though, and most of those resulted in a win or at least we put up a good fight. Sometimes there's nothing you can do and the match will just go to hell in a hand basket, it happens. Can't always win, nobody can. I still prefer team effort over individual carry promoting systems, even if it means that sometimes other people lose the game for me (or I for them). /shrug
I like the comeback exp because it makes turnarounds possible (among few others things like map objectives), you can never get complacent or afford too many mistakes even with a seemingly good lead, over-confidence can easily get punished.