Yeah, I was just thinking about that issue. I think maybe the ideal approach (assuming you're committed to using XP, that is) would be for the DM to set the XP value ahead of time based on a "default" party that just lumbers along, neither doing anything particularly clever nor particularly stupid. That way, if the group does manage to use clever tactics to split encounters up, then they implicitly get credit for that by still getting the XP as if they'd faced the goblins all at once. Conversely, if the party makes a ruckus and draws two groups of enemies together that wouldn't have been together normally, they pay the penalty of a harder fight, but without the reward of an XP multiplier.Both approaches have pros and cons.
The existing system rewards the same XP regardless of approach. Let's say you spot a group of a dozen goblins. If you can lure away half the group and kill them 6 at a time, the encounter will be much easier because the players approached it with a smart plan as opposed to brute force. Additionally, for "living worlds", the DM can have a patrol break off from the main group at given times, without having to recalculate their XP value.
Challenge based rewards discourage that style of play, while encouraging a kick-in-the-door approach. If you're dealing with players who really want to maximize their XP, they may even try to provoke multiple encounters simultaneously.
Either way is fine, provided it works for your group.