It might still be a little underwhelming, but at 9th level, they would also be adding an extra d8 damage for that attack from potent psionics gained at level 8. Also, if melee effectiveness is what the soul blade is after then they should probably be looking at disciplines that aid this, such as the brute force discipline. With that single attack they can use the knock back option and potentially deal an additional 7d6 damage if they knock them into an object.
Still, I'd like them to gain at least the base extra attack at level 6. I also don't really see the point of them gaining martial weapon proficiency.
On a side note, can they spend 7 points to gain the +4 bonus at 3rd level? The Psi Limit specifies discipline which the soul knife's Hone the Blade isn't.
I think that the Martial proficiency is so that no-one can say "but you are not proficient with your own soul knife", and the soul knife is a martial to prevent it from stacking with the monk (possible d8+dex dual weapon attacks at level 2 would have been pretty nice.)