why are esters bad?

I thought that belgian ales got alot of their character from esters so why does everyone talk as though they are a bad thing? perhaps I am completely wrong and that is why.

They aren't and they are, depending on the style you're shooting for.

Lagers = bad.
Belgians or "fruity" English Ales = good.

Esters aren't bad, but uncontrolled ester production is not good. Many beers depend on esters for characteristic flavors - as JetSmooth said, almost any beers other than lagers. Knowing that with choice of yeast and temperature control you can e.g., favor the banana ester over the clove ester in a hefeweizen gives you an edge in designing your beer.

Many yeast strains will overproduce esters at warmer temps. Once you reach 70F and increasingly above you will get more and more esters. While some yeast stains, such as the Belgian strains, produce enjoyable quantities of esters at warmer temperatures, others will produce a fruity mess. Hefe yeast at too warm of temperatures will go from tasty banana to gross bubble gum when it gets too warm or starts too warm.