to get into the business market, and it's mainly to that market that this offer is being addressed. The reason is that many small businesses tend to go for machines with fairly basic hardware specifications, and they (the machines) may not perform at their best running Vista business or Ultimate editions - the only two versions to which this offer applies.
People who bought these versions have always been able to downgrade if they wished, so there's nothing new about that - what's new is that PC manufacturers can now put XP discs in the box with the Vista machines if they want to.
Making the offer is one thing - how many people will take it up remains to be seen. Either way, it's going to be up to the computer manufacturers to decide how they handle this offer, and no doubt we'll get some feedback in due course. Personally I doubt that there'll be a landslide towards downgrading - the world's moving on, and XP home users in particular will gradually find themselves in a shrinking pool in terms of new software options. It isn't going to happen overnight, but it will happen.