The simplest explanation: No rule changes your game stats unless explicitly stated. So you take on the "form" of a dire tiger, even though you still have the same damage and attacks from the weapons you are wielding, can still cast spells through an implement, etc. You can even switch weapons, drop items, quaff potions, etc. So it's really just cosmetic.
Of course DM's Discretion can override the general rules in a particular context. So you might say that in tiger form you suffer a -2 penalty to checks involving manual dexterity, such as picking locks. But any general gain or loss of capabilities beyond what the power spells out explicitly should probably be discussed amongst the group.
-- 77IM