How about a +6 Con tome. even if you are rolling on a +4 that is still 28 HP and if your toon's only got a +2 you are looking at 56. Either of which is nothing to sniff your nose at.
And if you use this in a spot that doesn't have a tome at all (and there are still plenty of people out there in that boat) the benefit could be quite substantial. I know my AA-Assassin has eaten a +1 Int tome I found lying around that just put him on an odd INT... for him that +6 tome to int is +3 DC to assassinate straight up.
So yes compared to some of the rangen loot it's lack luster in certain situation but in many other cases if could be quite a boon.
As for power creep ... well I give up on it honestly, might as well toss in some +10 tomes even, I mean really that's only a +5 off of base and short of DC's that's easy to find for most other combat oriented things.