It would be very useful to be able to easily generate/configure an arbitrary testnet genesis block.
It would help a lot with regards to people pointing ASICs to testnet, ruining it for everyone else on one hand. On the other hand, if that problem is solved, it would allow testnet to behave exactly like main net (it would be possible to remove the weird "if no block has been mined for 20 minutes, accept blocks at diff 1").
That would make seeding peers impossible unless you specify them, but it would make testing so much easier in a lot of cases.