Oh, PLEASE NOOOOOO...............

[TsPoet]

First: I have 2 rescue mutts - both with baggage from the 2 years before I got them (not that I'm perfect), so I agree and practice what I preach.
But...
Lots of these designer mutts are intended to have a purpose. Almost all (all?) poodle mixes are designed to be more hypoallergenic, for example. Many are intended on being hardier/healthier due to diluting out breed-specific ailments. For example, a puggle is a beagle pug cross. It is intended to create a dog with a pug-like temperament, but with beagle-added health. Pugs can't handle ranges in temperature at all, and beagles can. On the other hand, beagles can be willful and hard to manage. The puggle seems to be a very successful designer dog.
I would love a puggle - I think pugs are cute and generally have wonderful temperaments, but I want a dog that can go hiking with me and can do agility.
I won't buy a puggle because there are several rescue dogs available at any given time that can go hiking with me and maybe do agility, depending on the complete rolling of the dice and the temperament I end up getting.
So far one of my two mutts is brilliant at agility, but we can't compete because he is fear aggressive and I can't guarantee he won't bite the judge and my other rescue has turned out to be extremely soft and scares too easily, so doesn't perform well under pressure. I love them both and will continue to do agility as long as we are all having fun and I wouldn't trade either of them.
So, my next dog will be.... a rescue mutt, and I'm going to name it Peeves - so I can tell everyone it's my pet, Peeves. But, hopefully that won't be for a very longtime as my two are happy and healthy.

Also, all current breeds were once designer mixed dogs.