Disney doesn't need to buy Hasbro, just putting new Star Wars films on screen pretty much secures their future with all the toys to come.
Disney does need to buy Hasbro as they want major control of kid's entertainment. They have the girl's 5+ market down pact with all of the disney channel stuff. They still don't have the boys market locked down.
Right now the Marvel and Star Wars brand will go a long way in helping secure this market. Disney has to be feeling pretty bad for letting go of the power rangers brand a few years back, they let the brand die while a well placed nostaliga campagin from nickoloden has revied the brand immensely.
Disney buying Hasbro gains them a lot of things. Highly valued properties in Transformers, G.I. Joe, my little pony, beyblade, Magic the gathering, and dungeons and dragons.
Not only that but the pruchase of Hasbro would be a cross production wet dream. You would have all these properties across what would become disney channels (XD, hub, disney channel) with disney now controling the mechadsing process for toys and such (instead of going through mattel and hasbro already does toys for transformers). Then you have all of the comics going though marvel instead of darkhorse (starwars) and IDW (transfomers and mylittlepony).
This puts them in a very good postion of printing money from one investement. They would be killing the middle man for the most part.
All disney would need now is a capable gaming divsion which probably wouldn't be hard to do. They have the lucasarts divsion as well as junction point. With a few smart aqustions and studio form ups (thq) they could control every thing.
Its scary and somewhat monopolstic and would probably be allowed to happen because even with that disney is still smaller compared to say news corp or time warner.