I don't know if the semis have a power advantage over the pures, but they do have an advantage in terms of story share, because they have something to offer in more situations.
As a semi (nightblade), I'm averaging 3-4 spell list ranks per level. Magic + martial arts + stealth does not leave a lot of points, but I have developed a lot of languages and sense/awareness skills. One paladin is averaging 4 spell list ranks/level and has developed a fair amount of lore and languages (she's a paladin of a goddess of knowledge, so...). The other paladin is more martial with only 2 spell list ranks/level, not a whole lot of skill breadth either.
In the RMU game, the one semi is the party shield wall (the rogue is an archer, and the rest of the party is pure or hybrid casters) so he has had to be very martially focused. A lot of points in adrenal skills, fortitude, footwork, etc. He's developed four spell lists to his level. He does have some ranks in culinary and survival, even a little social awareness, but definitely has to rely on the other party members for knowing stuff. That suits the character, who is a half-orc.