What I have experienced when we have a Widowmaker on the team more than once...
If a game is on OT and a Widowmaker is closest to the point, sometimes they are too focused on sniping instead of moving up to the point to reset the timer long enough for some of the other team members to get back. While situational and more of a communication problem, I have been in situations where the Widowmaker just needed to get in, touch the point then grapple back out so that the team can drop ultimates but sometimes you'll see her just standing on the landings or perches looking down the scope. Sometimes, and maybe they don't have a headset on, people have told or asked her to just touch the point and she doesn't. Meanwhile, the OT timer burns off and it's game over while a healer and tank are just shy of the point.
Some players just know what to do in ot regardless of what character they play. So I have seen a Widowmaker jump in to keep the game going and jump right back out when the team gets back.
As a person who hears groans when I select Mei or Sym, I understand. I don't have a problem with people picking whoever they want. I'm just giving an example of what I have seen and why people can get frustrated with Widowmakers on the team.