I would love to have the opportunity to give 5 upvotes for excellent elaborated posts even if I lost XP in exchange.
The problem is downvotes. For example, some people hate certain users, for example, sundialsvc4, and he gets modded down multiple times by the same user via proxy accounts. I've gotten it too, where it was obvious (specifically on two, maaaybe three occasions) that the same user modded me down three times. If mutiple downvotes were allowed, this malicious user, and others like him, would trounce the people they don't like.
Even if restricted to upvotes, there is an issue. The regulars like to upvote each other, perhaps unconsciously, and thus would be more likely to double upvote each other, making PM high ranked comments a mostly exclusive club of mutual back scratchers. That would be a tragedy.
Unless you think this comes up really often, i would suggest simply posting a "I really liked that post" post.