Seems its only BA who have said its a 2 bag rule for all now. BMI even split it 2 bags for business class and 1 for economy
Prior to the "one cabin bag" rule for all, most airlines implemented a policy of one bag for economy passengers, and two bags for premium economy and business, so the BMI rule (for example) is only going back to what they used to do prior to the imposition of the "one bag through security" rule.
In Economy (and certainly on airlines like FR and BE), there simply isn't enough overhead locker space/load capacity to allow people more than one bag.
However, the "one bag through security" rule played havoc with Business and PE passengers. I've just come out from MAN in Business on BMI, and there was acres and acres of overhead locker space. I'm away for 6 days, so I have a rucksack with the laptop/washbag/cables/camera/PDA etc, and a wheelie-bag with clothes in. On the way out (yesterday) I had to check the wheelie-bag. Coming back (on Thursday), I can bring it with me and therefore possibly stand a chance at making the 45 minute connection between me arriving and trying to get the MAN-BRS flight on WOW.
Whilst I accept that, by volume, BRS is now awash with LOCO passengers, it's worth bearing in mind that CO, LH, KL, and AF all offer Business products, and with that an expectation of carrying TWO cabin bags on board (or at least to the plane door!).