Subscription IPTV hasn't exactly taken off in the UK so far.
If there's a standard target platform in YouView and each broadcaster can run their own packages, rather than being limited to whatever channels BT Vision or TalkTalk.TV choose to bundle, then it could take off, especially if most new TVs have YouView bundled and so do many new FreeView/FreeSat boxes (especially all the PVR ones)
I can get Eurosport player for £2.99 a month to watch the cycling (niche market, yeah, but there's lots of niche markets around the world; this is the Long Tail in TV).
I can run that from a computer to my TV, but that's still a hassle compared to getting it to run inside a YouView box, or even a YouView app running in a media centre like XBMC that has a proper 10-foot interface and works with a remote.
It also will completely unbundle Sky and Virgin. If you can get any channel on YouView direct from the broadcaster, rather than buying a package from Sky / Virgin, then why not do just that?
Sure, the Sky channels are still bundled with each other, but it opens up a route to market for all the other channels. If I'm a minority sport where there's TV already being made (i.e. popular somewhere outside the UK), then all I need to do is take an existing English-language feed from another broadcaster and drop it to YouView - the subscriptions are pure profit. Even if there isn't an English broadcast, paying two commentators and a producer won't cost that much to slap a commentary onto existing pictures.