We traveled once in (late) September and most definitely did NOT need pool heat. In fact, we needed that pool to be very cool as it was so incredibly hot and humid outside... in the 90's every day. We were so miserable that I swore we'd never travel during the Summer months again. We'll be trying October next year... I'm hoping we don't regret it!
i vote no as well. i personally would only get it if going in Dec thru April (or a cold snap otherwise.)
also note that most places have onsite managers. if you got onsite and decided that you needed it, you might be able to contact someone and arrange payment so that you could get the pool heated for the rest of your trip.