*shrug* If all your limbs are embedded in a target's body and the target jumps, how can you hold yourself steady?

You can use as many limbs as you like in a full attack, but if you do not leave enough out to maintain some form of balance or control of your own motion, you fall prone at the end of the turn.

As per the OP, one limb allows you to maintain your balance/position, but you lose your dex bonus to AC and any related bonuses (dodge bonus, for instance). Two or more limbs allows you to continue acting as normal.