I honestly can't think of a piece of personal gear that is more worth the weight and volume to carry on a shoulder season river trip out West. If you have easy access to one, bring it. If you are buying and live in Eagle, you'll have plenty of opportunities to use it again if you don't sell it after the trip.
If you bring it, the worst thing that will happen is you'll have gorgeous weather and it'll ride down the GC in the bottom of your drybag or in the drybox. If you leave it behind and the weather gets nasty, there are a lot worse things that can happen, the least of which is having a miserable day (or week) on the river. When you're hot, you can always cool off between rapids with a bucket full of water. If you go for a swim on that one rainy day and are turning blue, shivering, and still 8 miles from camp it's a lot more difficult to change your situation and you've just become a liability to your group.
If I were the TL on a shoulder season trip, I'd require everyone bring one and hope no one needed it.
Rig to flip, dress to swim.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse