Smarten, I think the key differences here are the cost of a B ticket, the flyer's desire to continue to accrue US Airways miles, and (in my case) the availability of SWU's.

The original poster had access to B fares for $600 - upgradable for 20k. If he can do that, and therefore spend $600 and 6000 (net) miles for a Biz seat, and be 7000 qualifying miles closer to being able to do it all again next year, he probably considers that better than spending 80k for a Biz award seat. Change the number to from $600 to $3400, and clearly the decision is different. (I will agree that $600 sounds lower than usual for an int'l B fare.)

In my case, I don't think I actually needed a B to use the SWU's. I went 3 times to Europe and never paid more than about $500 per ticket - confirmed upgrades at time of booking. $500 + about 12000 miles vs. spending 80k? In my case, I was still accumulating US Airways miles, so spending the money and the SWU clearly made the most sense.