Fanaelialae
Legend
I don't see this as an undesirable situation. The entire point is that this is a last resort action. It's not meant to be used within combat for tactical advantage, it's meant to end combat for sheer survival.
Doing as you suggest changes the entire dynamic and function of the ability and makes it a viable once-off ability just to disengage and strategically reposition. That defeats the entire point of making the ability in the first place.
In that case, as I said, I'd suggest increasing the penalty to -5.
A -2 penalty to attack is almost meaningless when compared to encounter power nova. A -5 penalty will strongly encourage the party to avoid any further encounters in the day. A -2 penalty will not do much to discourage players to avoid using hit and run tactics against an already softened enemy group, though it would certainly discourage them from encounters with other groups of enemies (ones they haven't already beaten on).
EDIT: Also, your version is perfectly viable as a once-off for disengaging and strategically repositioning. If a player feels the need to use it, he'll use it (it's hardly as though your version costs xp or anything precious). It's simply that with your version, the player will likely want to avoid further encounters in the day (and because he's already used withdrawal once, there's effectively no further cost for using withdrawal the rest of the day to avoid all subsequent encounters). Whether Withdrawal is viable for tactical repositioning does not affect it's effectiveness with regard to running away from overpowered encounters in any way.
If you want something that can't be used for tactical repositioning, use something like the following:
WITHDRAW: FULL ROUND ACTION (yes, I'm aware full round actions don't officially exist in 4e)
Escape: You escape from the encounter.
Morale Penalty: You suffer a -2 penalty to attack until you take an extended rest. If you encounter the same enemy group (the one you withdrew from in combat) before an extended rest, you suffer a -5 penalty instead.
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