The facts: If you're able to get an agent, they will often work on the manuscript with you. Then, of course, if a publisher picks up your manuscript they will do their own series of edits.
It's true that more authors are hiring editors themselves before querying because they want to query a solid draft (or sometimes they've been in the querying trenches a while with no success and need the help).
Still, I like to be very clear about expectations so that you can choose what is best for you, especially when money is a potential issue.