Symbionts are more interesting than Hot to me. As interesting as the potential implications of having symbionts in a story/setting, i have yet to see any setting do much with them beyond a single character/creature.
Shameless plug: For those who like D&D and like it a little dirty, I actually made a race whos whole society revolves around symbionts. Its almost Zerg like in their usage of artifically created creatures to solve problems. Not to get into to much detail but a character of the race is encouraged to collect or "grow" more symbionts as it is a staple of the races culture. There is also a healthy dose of BE splashed in there. To cap it off I posted about a dozen homebrewed symbionts (stats and all) along side the race.
http://be-dnd-group.deviantart.com/ has all my stuff, Im Darktiki.