Some systems like laptops come with SSD drives only. They're typically around 256gb at the moment. 20gb is a lot of space, unless you're also lugging around an external drive.
What makes SPV clients have less privacy than full nodes?
In git master bitcoinj is now HD and so SPV HD wallets will start appearing in the near future. Garbling Bloom filters more aggressively is tricky and will take someone doing some serious work and R&D to really nail.