I just read a couple of answers on the same question where both users used Google Trends as evidence of word/phrase usage. The problem is that Google Trends shows search trends, not usage trends. Should we do anything about this or just trust voting to take care of it?
As long as the answer is clear about what it is, I don't see a problem.
If you see an answer saying it shows usage, I'd suggest editing it to say it shows "searches (which may be indicative of usage)". If the answerer objects and reverts, then downvote and comment.
It's a data point. Its usefulness is in the eye of the beholder, but unless there's a systematic problem with the data itself coming from Google, I'd say it's fine as-is.