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.