What if you rejected for no other reason than a too-low 50k word count?(though -oops-the author-doesn't know for sure it was this, God forbid an agent give feedback) would a revised 70k get the auto-dump as well?
There's a short answer to this, but I felt it deserved some discussion anyway.
At my agency, 50K will make me suspicious but I will not immediately throw it out, even though maybe I should. It depends on the genre; my boss is a little looser about word count. I know of at least two other agencies that absolutely would throw out a 50K novel, so maybe it's not a great thing to be pitching.
On the other hand, padding your novel doesn't make it good. It probably makes it bad (or worse).
There was a case a few weeks ago where someone sent in a query saying she had revised her novel to our specifications and now would we please look at it? As best as we can figure, she had originally sent a query (a partial or full we would have remembered) that one of us rejected, but written "too short" on the side or as a PS. Some agencies do this sometimes, if the writer needs a leg up, but in this case it came to bite us in the tuchus, which it usually does. She assumed this wasn't the only problem with the novel and spent a ton of time revising it, then resent the query. Rejected again - it was still a bad novel idea. I guess our (I don't know if my boss or I did it) helpfulness was misleading, making her think she had a chance if she added 20K of blather, or simply lied about the word count and hoped we really, really loved the partial.
I really hope, as a person, that she hadn't pinned her hopes on us. As I writer, I know she probably did.