I read an article the other day that really made me think. Not because I fail to follow the guidelines in that article, but because I try very hard to follow those guidelines. Well, not those ones, specifically, but basic, common courtesies that I was taught growing up and working in customer service jobs.
I'd love to give tons of examples of how following guidelines such as those will lead to success and glory. But let's face it, that comes with hard work, good timing, and many times can fall into the lap of the world's biggest jerk-faces. But if that's the case, why be professional?
The simplest answer: because no one likes to be around people who make them uncomfortable. If you're high-maintenance, rude, mean, selfish, or any other stereotype associated with the "diva" personality *cough* *cough* Prince *cough*, no one will enjoy working with you. And why should that matter if you're talented and rich and make those people (editors and agents) money? Well aside from the moral issues of treating people like dirt, this can be looked at in a selfish light. If the only reason people are working with you is that you make them money, they'll throw you under the bus in a heartbeat the instant the cash stops flowing.
But if you treat them well, there's a better chance (though not guaranteed) that they'll stick with you during the rough patches.
The best example I can give is my day job. I deal with people quite a bit, and some of them can really drive me up the wall. But most of the clients I deal with enjoy seeing me in the shop or behind the wheel of the delivery vehicle that has their equipment. I've built a reputation among the people I work with as someone who's going to work with the client to ensure that they have everything that they need. As a result, the clients will very often make sure that MY schedule is followed (like when we have gear that needs to go out early on the return date), and the other vendors I work with will help me stick to that schedule, and they won't gripe and complain if something has to be changed due to miscommunications.
In the writing world, agents and editors don't want to be around that one person who's only interested in selling a book. Sure, at conferences writers are hoping to develop a rapport with agents and editors. But I, for example, would never have learned that an agent who I would enjoy working with also LOVED the Avengers movie if I hadn't "turned off" the "will you seriously consider reading my stuff" mode. And we got to talk about Scottish music, traveling in Great Briton, books we both enjoy, and several other AWESOME subjects that had little to do with what she's looking for and what I write. And it was a great time.