This is what I've gathered in speaking with agents and from what they say at conferences and so forth. They're almost looking for a reason to quickly reject a manuscript. Some are getting 50 queries a day, and several are probably pretty good, and there just isn't time in the day to read/assess all of them. So if the chapters are not pristine, they toss them. This is just what I've gathered, and it can certainly vary from work to work (some ideas might just be too good to pass up) but my take is that it shouldn't leave your hands unless it's as ready for publication as you can make it.