Well, there are many reasons why hybridising is bad.
First of all, and the main reason, is that if the hybrids get out into the trade, and people don't know they are hybrids, they could be bred with pure strain fish. If that continues, there is a very real possibility that the pure breed could be lost.
Just look at what's happened with Endlers and guppies. There are now no pure Endler's in the aquarium trade. Imagine if something happened to them in the wild (pollution, or drought, for instance), if we didn't have dedicated people keeping pure Endlers, then they would be lost, forever; no more Endlers...they'd be extinct.
There's similar situation going on with synodontis catfish and Malawi cichlids.
As well as a species being lost, which is a sad, sad thing in itself, there's always the possibility that those fish could be useful to us; for pest control, or medicine perhaps. Think of how valuable the zebra danio has become in scientific research into heart disease, and things like that.
I know people will say, "oh, I won't sell or give any away", or "I'll always tell people they're hybrids", but what about those people; they might not be as scrupulously honest as you. Or (horrible thought) you might die in an accident and your family give your fish away. Imagine if they gave them to an aquarium or breeder and they didn't know those fish were hybrids.
I know it sounds over the top, and scaremongering, but these things are a very real possibility.