I dont think Sky always get the process right - rizla's comment of "although the Sky ordering systems don't show one is required immediately" may be the reason.
I think what may happen in some instances is that Sky dont do the SPO process properly. Ive known this happen for both my daughter and my friends daughter, both experienced broadband delay.
What actually happens is that Sky take your order to move without a MAC and the order process for the phoneline move gets generated; after a couple of days the order system updates and (if you were on SMPF) then you have a section where you can enter a MAC.
If you don't enter the MAC then Sky move the phone and then do BB after the phone is moved. You will go directly from SMPF to MPF and will lose BB for a few days. Edit - this is usually the point where the router isn't sent out on time as well.
If you do enter the MAC then Sky will use that to move your BB and then some weeks later will move you to MPF - probably when they have a batch of jobs to do on same exchange. You won't lose BB for more than a couple of hours.
I think this is because if you give the gaining ADSL ISP a valid MAC code then they must use it. Never actually checked that however.
It costs Sky more money anyway, simply because you stay on SMPF longer. I'm sure they'd rather you didn't use a MAC.