The one month option is pretty choice for some people I'd wager.
I think paying a yrly sub forces me to play more games as I dont want to pay for nothing.
Its one of the main reasons Xbox was an easy no for me.
This one month thing lets people try it out without feeling a need to play every waking moment to get their moneys worth.
Thats how I feel about it anyways, and I have had plus since it launched and figured this out the hard way.
I dont have plus anylonger, when ps4 comes out Plus' value will increase while remaining the same price so I'll prob jump back in for a yr and see if I still feel the same.