Oh dear, with XMR, XDN... and other Cryptonight coin I always use: transfer 0 amount , i don't know on subtoshi must have -p. Let me try.
Thanks you.
The -p shouldn't be central to subtoshi, but rather a change in simplewallet. Also, thank you arielbit for pointing this out.
XMR and XDN 'transfer' command have only payment_id option and that's why they don't need a flag '-p' for payment_id. Dashcoin (and Bytecoin) has also transfer_fee option (-f flag). If XMR and XDN add transfer fee option (or any other option) they will be forced to change the usage of payment_id.