I think it's a spoiler to drive up the bid price and make like difficult for IAG. I just can't see Virgin having the financial muscle to make a go of integrating bmi. It's telling that Virgin seem to have only got their act together after the IAG announcement. Also remember neither bidders have finished due diligence so they don't know yet what exactly it is they are taking on.
I don't think IAG will take a counter bid from Virgin lying down either.