There are always lots of conventions at Swan/Dolphin, all year round. They are convention resorts & can be full to capacity because of that. We've been to conferences there in May & October. December 2007 was sold out at The Dolphin because of conferences while we were there.
I've never booked a room higher than standard & just always hoped for an upgrade. In 2007 there were no upgrades available, because the hotel was full. One year we got upgraded to Resort View, last year we got upgraded to Resort View with Balcony. There are different levels of "free upgrades" depending on what level Starwood member you are. For example there are no suite upgrades available for Preferred & Gold members, but possibly for Platinum. Awhile back someone posted the different upgrades available for the different tiers of membership. I would assume you could get a room with a balcony with your membership, but I wouldn't expect the Presidential Suite!