The problem with that approach is bookkeeping. It's all very well if there's a handy cleric on hand with Restoration tucked away and prepared to cast immediately after the battle, but the complication is if you level up in the meanwhile.
For example, if a group of 6th level characters are fighting a wight and some assorted non-draining undead. During the battle, the wight manages to drain one of characters. However, as a result of the battle XP, all the characters are able to level up (including the character drained to level 5). So, the drained character is back to level 6, and the other characters are at level 7.
Now, the cleric casts Restoration on the drained character. What happens? Does he go up to level 7? Does he go midway? Does the spell fail?
The first offers an XP loophole. If a character with 55,000XP (just hit level 11) is drained to 50,000XP (midway 10/11) and then levels up again (so now has 55,000XP again, one level down) then a restoration would take him to level 12 (66,000XP). Net gain=6000XP.
The second doesn't make sense at all. It has no justification to back it.
The third is both nonsensical (the level drain has still been experienced: why can't it be restored) and harsh.
Thus, the best way is to register HOW MUCH XP has been lost. In the above example, 5,000XP was lost. When restored, the character recovers that amount. Simple.