I think Widow is a great offensive and defensive hero. You're right that she can't be on top of the actual objective, but you can certainly pick off individuals hanging around the objective and when you take out something such as a lucio, it can be game changing.The key is to know how to navigate the map and play smart.
Plus, in my opinion, her ultimate is great. Probably my third favorite pick behind soldiers and lucios ultimates. It's easy to use and basically impossible to misuse. I actually much prefer it over Hanzos, (I never get more than 1-2 kills with that, I'm also just not a Hanzo guy though).
Also, while her assault rifle isn't the greatest, it is definitely usable and I've killed a plenty of heroes with it, especially when combined with the mines or a charged shot or team shots.
However, I will say that a decent DVA that wants you dead, will have you dead. The new DVA will maul through you if you're on her hitlist.