ToME WikiI don't know if it's necessary to improve the Backstab skill - certainly I haven't played the assassin enough to test it. From what I've seen on the forums, it's been suggested that Backstab does nothing more than a little bit of extra damage when the fight with a monster starts, and a little bit of extra damage when the battle has already won (unless you can inflict fear).
Here's a suggestion: In addition to backstab doing extra damage to sleeping and fleeing monsters, have it also deal extra damage to confused, stunned, poisoned, wounded, etc. monsters - each with its own varying modifier. Stunned would probably be 2x (as with sleeping), confused maybe 1.8x, wounded 1.7x (as with fleeing) or scaling upwards depending on wounding (if monsters have the same graze/cut/bleeding system), poisoned 1.5x. Hm, what other status ailments are available?
The idea is that this leads to a little bit more strategy - when you've invested enough into backstab, it will be worthwhile to use weapons that cause bleeding or poison, or perhaps bring around wands or staves that cause status ailments. The question is, of course, if that's possible at deeper dungeon levels, but that's another issue completely.
Another idea: I recall in Icewind Dale you have the option to replace backstab (which, as in most other games, comes with a nice healthy multiplier in the range of 5x rather than a paltry 2x :P ) with Crippling Strike. Again, I don't know if this is useful in deeper dungeon levels, but it could inflict a random (or selected effect, by the player or automatically targeting the monster's strength) stat or speed drain. Thus it could slow down a monster when you cripple its legs, reduce the strength of its attacks by hittings its 'arms', drain intelligence or wisdom with a strike to the head (perhaps with a chance to inflict confusion or stun).
Perhaps Crippling blow starts off as a way to reduce speed, scaling up with skill: speed = -(S@20), giving you the option to reduce strength instead at level 15: STR = -(S@10), and the option to drain intelligence or wisdom at level 30.
Or perhaps crippling blow starts off as a simple backstab, and allows you the option to drain stats instead of dealing extra damage.
I think it's sufficient to pump up the damage. The damage should be calculated as a bonus rather than a multiplicative factor so that backstab is useful at early levels as well as late.