When you wander around in, say, WoW, with the possible exception of PvP areas, there's almost no such thing as a surprise attack. You can see your opponents, attack them first, even avoid enemies you know will trounce you. This lends itself to a more planned, bloodless approach to combat. Sure, it's still fun, but think for a moment of the challenge of being suddenly beset by a hidden enemy.
Let's say you were harvesting some Peacebloom, no enemies in sight. Your guard is down, you're happily gathering away, you may even be whistling a little tune. Suddenly, bam, a giant Venom Web Spider drops on you from above, taking you by surprise and getting in a few hits before you are able to effectively respond. A few things happen here:
- You start swearing
- You might startle a bit
- You put your macros to good use
- You experience the rush of adrenalin that comes with true combat
Or consider raiding with your guild. Exploring a new instance can be just as anxiety-riddled a prospect as anything, but after a few run-throughs, you can assemble your game plan and come up with a workable strategy. After that, it's about the proper application of maneuvers and math. You know how each enemy behaves, where it patrols, its range of awareness, etc. Imagine how much more fraught with peril an area would be if you had no idea, moment to moment, where anyone would be, and in which numbers.
Think about the infamous Leeroy Jenkins debacle -- what if they'd have been able to pull it off anyway, taken by surprise as they were? It's this off-the-cuff excitement I think random battles would bring to an MMO. Sometimes it's fun to be surprised, not only by your enemies, but also by yourself. When Concerto Gate arrives, rather than dismiss it out of hand, let's see how it all plays out. It may provide a worthwhile challenge to an increasingly well-trod mechanic.