I'm tempted to put a rear rack on my commuter....but I'm not sure if I'll always want it there. It would be nice if there was a simple way to snap it on and off without the use of tools.

does anything like this exist?

Yep...topeak seat post rack

https://www.probikekit.com/cycling-ac...FWNp7AodHGkABw

Mine had a quick release. Take it off in 5 seconds, put it on in about 15 seconds.

Once you've adjusted the rack to level on your bike, you need only a single hex key and 3 minutes to add or subtract a rear rack from a bike.

+1. It helps to keep the bolts in a baggie taped to the rack, or some other method that ensures you won't have to hunt for them.

I can get my Tubus racks (front and back) on in 10 mins; off in less.

Yes that is a great idea, but it only holds 15 pounds of stuff, which is great if all your doing is commuting to school or work, but not so great on short or long tours where more stuff is needed to be carried. If you need something more substantial to carry 45 pounds of gear or so then the a Tubus or Jandd racks are the best and can be taken off in about 5 minutes. However both of those racks will set you back about $85, if that's too pricey then Topeak Super Tourist racks are pretty darn good for the money and only cost $45. I believe in getting a rack that is over rated for what you need because a rack rated for 45 pounds does not mean it will carry 45 pounds because the bike as it travels is bouncing the load which could generate load forces of 70 to 90 pounds, and after a prolong bouncing condition the rack could break. So if you know how much weight you'll be carrying then find a rack that can handle double that amount.