I think so, its high time.... for all practical purposes, its one country... and one country = one team!!
The plus'es will include a stronger EPL (specially with celtic and rangers joining in), larger base of talent, better chance for some great talents like Best/Giggs/etc of the future to make an impression in the international stage, and some good local coaches like SAF!
Also, together they would have at least one team qualifying for the major championships!! (sorry, couldn't resist that one!!)
Negatives - can only think of one - it undermines the local unique-ness of Scotland/Wales/N Ireland... some of the international/CL exposure than these guys get will reduce... less CL spots, etc.
But overall, I think it will be good for football.
What do you guys think?