It's a double edged blade.
Someone telling you to fix grammar and spelling, unless they go through and point out all the mistakes, isn't very helpful. If they do though, you should thank them because its hard and mind numbing work.
On the one hand, its not helpful unless it's a final draft. On the other hand, if it IS a final draft, it does have to be perfect if you intend to sell it. If you send something to an agent or editor with those kind of errors, its an automatic no.
I, personally, never comment on grammar or spelling. I'm shite at both. So I LOVE it if people point out specific errors.