This is already an improvement. Back in spring 2003 (April, methinks) I booked a BMI r/t flight and it would only accept the BMI Diamond Club number. Now at least EuroBonus (which I'm using) is supported.
This lead to a funny confusion about a checkin agent in Dublin being "absolutely positive that your number is in the system". Which it of course wasn't and I ended up having to send in the stubs for mileage claim.