Simple explanation – it brings iWork in line with iLife and OS X itself – neither of which prompt for serials.
I have a suspicion that Apple would actually like to make iWork part of the basic Mac bundle in the same way that iLife is – i.e. iLife helps generate more money selling Mac systems than as a standalone application. The problem is, that would directly offend their friends at Redmond – they do not want to push them into cancelling MS Office for Mac.
(The funny thing is how little most Mac users are aware of the level of basic Word support in the built in TextEdit)
As for OpenDocument support – while it would be nice, iWork is commercial proprietary software. The MS Office support is there because it commercially makes sense, but it’s not as if there is a lot of real (economic) demand for OpenDocument support yet. What would tip Apple’s hand with OpenDocument will be public sector institutions insisting on OpenDocument support / documents.
Bear in mind that PDF is considered an acceptable open standard by most institutions for read-only documents, this would only be required for editable/collaborative documents.
I also suspect it would suffer the same issues as existing MS Office support in iWork – i.e. Word does not support all the same layout features as Pages, which is closer to a DTP package in some respects (actually, closest comparison would be MS Publisher, which even MS haven’t included in OOXML).
I’m unsure as to what degree OpenDocument is better than OOXML in that respect?