I've been developing on appengine for over 2 years now. Thanks to the platform, I've never been worried about traffic spikes. Whether you get profiled on TC or a celebrity tweets about your app, you know it will run. Given, you would not learn admin skills, but you'll learn a lot of other things.

I learnt how denormalized data in the datastore can help speed things up for your app. I learnt sharding thanks to app engine. The pricing has made me make use of memcache more often and trying to avoid hitting the datastore. When I look at it, I feel I've brought in more discipline in my code because of app engine. I am now in the habit of building APIs that would run instantly. If a http call takes too long, my instinct is to make it run as a background task and then return the result.

I've learnt all of these because I've been on App Engine. I can see a significant improvement in what I'm building now. The apps we build now, all our users say it's "fast and responsive". A lot of that credit goes to App Engine and the things it has taught me.

Nothing you learn in life is really a waste of time.