Pure gas is getting harder to find but it is still available. Should I seek it out? Why or why not?
A 10% ethanol blend will theoretically reduce your gas milage by about 3% (this is one post where I get to say YMMV and really mean it!)
On the other hand ethanol acts as an oxygenator and helps reduce carbon monoxide and several other tailpipe emissions.
On balance, probably a little better for the environment, neutral to slightly negative to you personally.
Any opinion of what it does to seals, rings etc?
On modern cars, nothing.
On my old 1964 Oldsmobile daily driver? It ate away at the rubber needle and seat in my carb about every 6 months or so.
I have had two cars run on 10%; both seem(ed) to run better on it.