So out of the boosters I've open so far, I'm only really missing two (Jedi Temple & Mos Eisley Spaceport). Is there any reason to keep more than one of these battlefields in my set? I have a few triples now.

I was wondering the same thing. I collecting with my brother, so I figured we might as well keep 2 of each, but after that I see no need to.

two of each for me, in case someone is building a deck to play me using my collection. That way they can choose in secret.

I think many are commons, right? So yes, we will have extras.

You can only use one in each deck, so one is enough. If you want to make multiple decks out of your cards it might be handy to have extras - entirely up to you on that.

I tend to keep a large number of decks built at any given time, so the multiples help there. That said, I've yet to find an overlap case, so...

It's a nice thing to have if you want multiple self contained decks, but you only ever actually need one.

you don't really need two for anything specific. It's sometimes nice to have extras though for multiple decks or things like that.

A full playest of cards involves 1 of each battlefield, 3-4 of each non-unique character and 2 of each for everything else. This will give you everything you need to guarantee being able to build ONE deck at a time. Having extra cards OR avoiding duplicate choices between decks allows you to build more than one deck at a time. In that sense, extras are really good to have.