I think there had been discussions about this previously.
Firstly, not everyone will take up their entitlement - people who already have enough heads (I'm inviting a comment from Scotty here, aren't I?), say, more than 5m shares, probably wouldn't bother (they're probably SIs anyway). People who have less than $5k invested in GGP probably don't have spare cash laying around to participate. Some people just can't be bothered mailing cheques in, fill in the paperwork, etc.
Hence there were a few warnings from various senior posters here to think before selling your heads just to participate in the SPP, as you may be disappointed with your allocation.
But I agree, the announcement isn't really that informative...
GGP Price at posting:
3.7¢ Sentiment: LT Buy Disclosure: Held