First of all, build experience, not hours. Don't even get me started on that.
Second, was your instructor an ag pilot? If not, then he probably didn't have a clue what he was talking about. If he was, then had he any experience interviewing for airlines with an ag background? If not, he still probably didn't know what he was talking about.
Yes, an ag background carries with it a certain stigma. Not the least of which is the fact that if you spray long enough, you're probably going to wrap up a powerline one day. Or a tree. Airlines like a clean flying history. I don't know any old ag pilots who don't have a few mishap stories...doesn't matter how good you are, or how experienced. It only takes once, and there are many opportunities when performing ag work for it to happen that "once."
Ag flying is not entry level work. Airline flying is more entry level, but ag flying is not. Most operators won't touch you without at least a thousand hours of ag, and therein lies the catch 22.
Cowboys? Not hardly. I don't know any cowboy ag pilots; but I do know a lot of ag pilots who are dedicated professionals, and who can fly the living tar out of an airplane in a safe, dedicated, professional manner.
Flying ag you had better know how to fly, because you have everything already against you. You're low, you're slow, you're loaded to the gills, you have no additional power or performance, and you're up against obstacles, winds, etc. And the work is extremely precise. Being off by a foot or two can spell the difference between lawsuits or refusal to pay, and the survival of the company. Very precise work, with long hours.
I worked with an ag pilot I know to get his instrument rating the year before last. He has been spraying most of his life, but has been putting off the instrument rating...no need. A need came up, and I talked him into getting it. He was intimidated, but I've never seen someone who flew the airplane so naturally, or who took to flying instruments so quickly as he did. It was intuitive.
It's worth noting that the test which the State Department uses when screening applicants to fly ag airplanes against certain illegal substances in the southern hemisphere is an instrument test, in an ag airplane. They want pilots who fly exactly and precisely, and who have ample ag experience. If you ever have the opportunity to sit among a crowd such as might gather there, or elsewhere, you'll quickly come to realize that it's no group of "cowboys" at all, but a group of professionals who operate in a little different manner than you might. But professional none the less.
Pilots with an ag background do get hired by airlines, and other parts of the industry, too.
If you're looking to ag as a time building gesture, then give it up right now. Go flight instruct.
If you're willing to learn about chemicals, insects, and husbandry/farm work, then go for it. If you're prepared to work on the airplanes (you break it, you fix it), if you're prepared to do what it takes to get the job done, and if you're prepared to do what it takes, go for it. It's not easy to simply walk right in and go to work. The industry is shrinking, dying in a sense. The work is still there, but today it's often being done by larger, faster airplanes, with fewer and fewer openings being made available. Of those openings, very rarely is one given to someone without substantial ag experience.