Isn't this obvious? They keep a few mages around because some are useful. The key word being some, rather than all.
This is not the 'all or nothing' situation you'd like to present.
The Dalish have competitions among the candidates to find the most suitable, and dispose of the rest. Sometimes that means giving their extras to other clans, such as in the case of Merrill, and sometimes that means banishment.
I'm sorry you're bitter that your preferences don't match what the writers want for their world, but you're either going to have to get over it and accept, or move on.
As I've explained already, I'm perfectly happy to change my views when a development makes sense. In this case, it doesn't, so I'm stating my opinion as such, and that if this is how things are in the Dalish clans, my opinion of them has lowered considerably. I'm not telling anyone else what their opinion should be or to "move on."
Once again, Merrill wasn't an extra. Did you see any spares running around in the Dalish origin? She was loaned out because they didn't have enough, not because there were too many.
Basically the same reason Andrastian countries keep mages around. They, however, have the resource, Templars, Seekers, etc, to combat magic gone awry on a large scale. The Dalish don't.
Once again... (would people please read the points already made?) mages are perfectly able to use spell negating magic and no templar can come close to what a powerful mage can do against another mage. The templar order is just a way for Andrastians to convince themselves their holy warriors are "pure." The Dalish have apparently absorbed these superstitions.