I've had the valve seal leaking on my truck for nearly a decade. Other than the small amount of oil that leaks down (while it sits) and gets burned up on startup it's nothing I care to fix anytime soon. Just have to put more oil in it, and it probably shortens the lifespan of the spark plugs a slight bit. I also have no cats on my truck, so the small oil burn can't effect those.
I suppose if you've got cats and don't get the car up to operating temp to burn the crap off the cats over time it could cause a problem.
Cosmos Black/Black 1995 540iA M-Sport
BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build