Im not sure why the devs wanted to add this before launch, I would have been happy to wait for it to be added later but it is a good feature, but i should also mention that there are other coins currently already using it.
1) devs will be in control of a huge amount of coins and do not want to risk holding them in regular accounts.. 1 hack = devs lose credibility and nem dies.
2) other coins do have multisig, but not blockchain based multisig. there is a MASSIVE difference.
3) the more skeptical investors, or risk averse, will feel much safer investing in Nem knowing they can secure their investment safely with multisig.
4) 3rd parties(exchanges, payment processors et al) will be able to use multisig to secure funds so we wont have any mtgox, bter, bitstamp [and so] type hacks which make the coin stolen look bad even though its nothing to do with the actual coin(most of the time). also we wont ever end up in the position of nxt having to think about a roll back to recover 5% worth of coins that got stolen.
having multisig ready for launch was the BEST move the devs could have made.
While that is the ideal pov i do want to add some thing from my pov.
@2) I maintain that NEM is NOT the only system with blockchain enforced multi-sig. BTCs multi-sig has to be enforced by the blockchain. It may be right that you have to prepare everything for that on the client side but the blockchain has to enforce it somehow otherwise there is no multi-sig. If I have multi-sig set up and the network still accepts tx without the co-signatories then noone would dare to say BTC has multi-sig because that would be a lie. NEMs implementation is infinitely more userfriendly though.
@4) They have the possibility but I personally doubt that they will use it (eventhough they very much should). From my experience crypto-service providers have their one implementation and try to squeeze every new coin in there no matter what. Everything else is just to much of a hassle for many or even most. At least that's what I suspect.