I have never been a fan of on-court coaching, but I have tried to be open-minded about it. There are just too many things wrong with it, though, for me to support this trend; indeed, I want it to go away.
One of the main arguments against on-court coaching is that it is the antithesis of what tennis is all about--the challenge of one (in singles) individual--alone--figuring out what to do against an opponent. Though some may perceive this as a purist concept--so what? The image of the tennis player on her own, mentally and physically equipped to make fast decisions and execute well-timed shots, is an image that suits the sport.
Then there is the gender issue. The ATP does not have on-court coaching, so having the women get help makes them look especially dependent. And--as Tom Tebbutt commented last spring (the editorial is, unfortunately, no longer available) in The Globe and Mail, it is disturbing to see middle-aged men--often fathers--coming onto the court to tell women and girls what to do. I agree. And while I know that the Sony Ericsson WTA tour gives lip service to equality, when it comes to action, sexism is the order of the day. Having older male authority figures come onto the court to direct young females is a disturbing image indeed. The fact that there are so few female coaches (that's a discussion for another time) doesn't help.
The Sony Ericssson WTA Tour defends on-court coaching by saying that the "fans want it." What fans are these? I have never met anyone who watches women's tennis and likes on-court coaching. And even if such fans do exist, pleasing them via a system that makes women look weak and tennis look tacky should not be an option.