I understand that the payment ID is stored in the tx-extra field. Only one payment ID can be specified when sending, even when sending to multiple addresses, because it's specified on a per-tx basis.
I'm assuming that it was made this way to preserve space. Specifying it on a per-output basis would mean it being repeated many times for each denomination even for the simplest transaction to one address. Was this the only reason?
With ringCT, would it make sense to consider enabling payment ID on a per-output basis? Was this considered, and if it was, what would be the main pros/cons. I take it that with ringCT, we would have to repeat the pID 2 times (output+change) if we want to prevent leaking which output is which.