Witchdoctor spells can be used with any weapon. This isn't a bug, as far as I know. Most of the other classes have buffing spells that can also be used with any weapon. The only class that is really restricted, as far as I know, is Musketeers, who need a Shooty weapon to activate their ranged attacks. However, I'm pretty sure Musketeers can also use any of the pistol/blade combo weapons, in addition to dual pistols and muskets.

The tradeoffs to using other weapons might not be immediately obvious, but they do exist. Staff weapons use Will, which is also used by Witchdoctor spells. You can focus on gear that adds more Will, and get more damage from *all* your attacks. All other weapons use a different stat, which means a Witchdoctor would have to choose between bonus damage for spells, or bonus damage for normal attacks.

Staff weapons also affect the range of Witchdoctor spells. Staff weapons that only hit a single square usually have higher range, which greatly increases the range that spells can reach. The only other weapons that do this are the muskets (the shooty weapons that look like rifles) and I'm not sure they affect the range of non-musketeer attacks. I've never tried to use one.

Finally, the weapon your character is holding changes the way the Juju series of buff powers affect you. If you're holding a Staff weapon, those powers boost Will - again, giving you bonus damage for all your attacks. If you are holding a blade or gun, that power will boost Agility or Strength. Meaning you'll get bonus damage for your weapon attacks, but *not* for your spells.

The only real advantage for a Witchdoctor who chooses to use a blade is that they can move and attack in the same turn. To get that advantage, they're giving up additional range and possibly bonus damage for their spell attacks. And since Witchdoctors can't naturally wear armor and don't get bonuses to Dodge, they're also risking taking extra damage from melee attacks.