Content and prestige
The advantage that Sony has is that it controls a lot of content, and Sony's products are viewed as being prestigious.. maybe not as much as Apple products and heaven knows that they've done a lot over the past few years to screw their reputation up, but still a Sony is usually a nice thing to own.
In terms of content, they have the stupidly named Qriocity service, plus music, TV shows, movies and then they can continued to leverage sub-brands and technologies such as Walkman, Playstation, Cybershot and Bravia (there have been Bravia phones in Japan). Sony will add the Xperia sub-brand and then cross-sell like mad I guess.
Back in 2001 the JV was the way to go. If you remember that far back, people really did just buy mobile phones and at most a couple of ringtones and some Java games. Ericsson was probably a stronger brand those days in that market, so it worked. These days people buy a whole experience, and really Ericsson is a B2B outfit now and has very little consumer engagement.. where Sony is very consumer oriented.
In those ten years, they've had some pretty influential handsets. The article mentions the P800 and P900 (and I guess P910) smartphones, and the T610 looked brilliant.. my personal favourites have to the the W800 and K750. After that it went into a decline until SE upped their game with the latest Xperias.
My guess is that the Ericsson name will disappear from the handsets after the deal closes in the New Year.