There are many benefits to 15-man raids that work around the scaling problems of 25-mans and the tuning issues of 10-mans. The big issues with 25-man raids include scaling and filling out the roster. Many guilds cannot field 25 people for their weekly raid and usually have to resort to pugging a few stragglers who aren't in tune with the group. Other guilds who cannot field 25 players choose to do two 10-man raids, but the issue of group composition takes center stage; where two 10-mans require a total of four tanks and five to six healers, the 25-man requires two tanks and six to eight healers. You can get a nasty split if you don't have just the right number of specific classes.
10-mans have the benefit of being an upgraded group in that the regular group dynamic of one tank, one healer, and three DPSers is just doubled (in favor of one extra healer, usually). The biggest issue 10-mans currently have is the class distribution issue -- it is hard to accommodate every class in a 10-man, especially in heroic encounters, making group makeups and careful, specific group fixing the norm in order to progress.
15-man raiding potentially solves the issues on both sides of the coin. With 15-man raiding, you would have less of a burden to field a group consistently, as well as more spots for more classes. Also, Blizzard would have the ability to design encounters around a three-group setup, each with a tank, three DPSers, and a healer. There is a natural progression from 5-mans to 10-mans and potentially in 15-mans that doesn't appear in 25-mans because of the odd group configuration.
Anyway, it's all speculation and just fun to think about. There is no indication that 15-man raiding will become a feature in the future, but there are definitely plusses to the system in terms of design. We haven't seen a Dev Watercooler from Ghostcrawler or Fargo in a while, and I would love to hear their thoughts on the subject from the design point of view.
The news is already rolling out for the upcoming WoW Patch 4.2! Preview the new Firelands raid, marvel at the new legendary staff, and get the inside scoop on new quest hubs -- plus new tier 12 armor!