I've bought a few B fares in last-minute, emergency situations. Sure, the price hurts. And while Z fares may be roughly as expensive as B fares, they do not provide any flexibility (not refundable for example, unlike B fares) and anyway they're not available for last-minute trips.

See the bright side of things: you will get 150% EQMs, and you could get a very cheap upgrade if you choose to use miles to upgrade. If you fly approximately 5,000 miles to Europe, you'll earn 5,000 * 2 (return trip) *2 (1P/1K bonus) + 500 (online booking) = 20,500 miles. The upgrade would cost you 15,000*2=30,000 miles. So, the net cost is less than 10,000 miles for two transatlantic upgrades, with the flexibility of a full fare. Not bad. Of course the upgrades may or may not clear, but the B fare will definitely help you on the DM list. Oh, and if you use the Platinum class UAL Visa, and the ticket costs, say, approximately $4,000 you'll basically get the upgrade for free.