A bit of a risk for WWE to re-sign a guy with such a history of troubles and "personal demons," especially at a time when WWE is probably more concerned about their image than ever before.
Having said that, nothing ventured, nothing gained. What's the worst case scenario? He goes back on the booze, gets fat again, starts no-showing events again, and he gets released again. No real damage done to WWE, and he's burned his bridges for the final time.
Best case scenario? He keeps his shit together for a while, which has potential to be interesting in some form of interaction with HHH and HBK. A natural feud between him and Carlito who he accuses of being a cheap rip-off of Razor Ramon. Possibly even lures Nash back to WWE (I don't know how dedicated he is to TNA or what his contract situation may be).
WWE has gone out on a limb for people before. They have given Marty Jannetty multiple second chances, which didn't really work out, but they were willing to try. They reincarnated Tatanka, they were willing to bring Golddust back a while ago, Jim Duggan is still kicking around, etc., I've not seen any recent pictures or have not seen him wrestle in a really long time, but assuming he's in reasonable shape and able to go, why not?