The owl tomes seem a lot less practical since it requires all of your team to be stuck together. Compared to blade tomes, where you can get good buffs from a single ephraim, eirika, etc, while the other two units can deal with/bait other threats. The upside I can think of though is that it works off of positioning instead of on how your team is composed to support the single blade tome user, lending some more freedom of team skill composition. But since it's not true damage, you might need spur skills to compensate anyway for getting off color kills. Conjectures.
e: hm a blue tome flier/horse could be pretty crazy bulky with only one adjacent ally with the respective fortify skill, that's +8.
e2: with the new fortify res seal, ephraim/eirika can now by themselves buff all four stats of an adjacent (-blade) ally. Insanity.