Off the top of my head:

1. Your skill in a weapon is part of the determination of how well you parry that particular weapon. If you don't practice dagger, then you won't be able to parry daggers as well as if you had practiced them. Not that daggers are all that hard to parry, but still...by not practicing, you give a dagger-wielding enemy an advantage over you.

2. If you don't know daggers, the poison dagger skill will be useless to you. Probably not that big a deal, but I've found it useful in the past.

3. Assassins don't have weapon-based skills per se, in the same sense that warriors do (such as jab, cranial, pincer, etc). But just like every other class in the game, they benefit (do more damage, can get a weapon that exploits a vulnerability, parry better, etc.) from using weapons. Dual wield is a powerful skill to have when you want to go on the offensive against someone that you don't need to use a shield against (mages, for example).

4. Once assassins get unarmed defense and backfist, fighting certain opponents unarmed can make sense. Before those two skills, you're better off using a weapon in pretty much all situations.

5. Assassins have plenty of skills in their toolbox, so you could probably play an effective hand-to-hand only assassin against certain opponents in certain situations - but you would have to be pretty skilled to avoid dying in all the other situations you'll find yourself in.

My recommendation: practice all your weapon skills, even if you don't intend to use those weapons, unless you have a really solid RP reason for not practicing them. Then go ahead and use the weapons. And avoid duergar axe-spec warriors (the bane of assassins everywhere!).