Here's my motivation for /P2SH/ or something like it:
I want to stop playing whack-a-mole with wallet stealing viruses and trojans, and I think requiring more than one private key to sign away your bitcoins is the critical feature needed to do that. Keep one set of keys on your computer, another set of keys on your cell phone, teach each to talk to the other before sending out bitcoins and you're safe (as long as a virus or trojan doesn't infect BOTH your cell phone and your computer at the same time).
The bitcoin protocol already supports that, but the bitcoin network, the bitcoin software, and the bitcoin addresses that we're all using now don't support multisignature transactions.
OP_EVAL and /P2SH/ and Luke's OP_CODEHASHCHECK are all slightly different ways of implementing multisignature transactions that are as short as the bitcoin addresses we're using today.
RE: the timeframe:
I'm pushing this hard because I'm tired of hearing that users lost their bitcoins to trojans and viruses, and getting there is a multi-step process that will take a lot longer than I'd like:
1. First a majority of miners have to validate, accept and mine the new transaction types. (that's the Feb 15 date)
2. Second we have to convince enough people to upgrade so that they are relayed around the network and not dropped
3. Finally, we can release software with wallets that use the new feature.
I'm losing patience because this process started in October, over three months ago, and certain people seem determined to do whatever they can to derail it-- if I was more conspiracy-theory-minded I would think somebody was purposely trying to keep bitcoin less secure than it can be. roconnor brought up legitimate complaints with OP_EVAL that were discussed and addressed with /P2SH/, but I can't respond to every half-baked scheme that is supposedly "better" or I will spend all of my time explaining why something like CODEHASHCHECK is a bad idea and have no time left over to making Bitcoin better.