For me I kinda think that this fragmentation is alright at this point of time. Nokia is pretty much betting their life on the WP platform, which explains why they are spending so much effort and probably money into luring in developers and apps on the platform. So it stands to reason that they would want it to be exclusive for a while. Samsung and HTC are great manufacturers that can put out awesome hardware, but I can't help but feel that Android is their primary concern at this point. WP is like the ignored step-child that they have to patronise. Face it, Android is the OS that is bringing in all the money for them. And, its not as if that Samsung and HTC aren't doing exclusive apps as well. Its probably just that their exclusive apps are seemingly not as desirable as compared to Nokia's offerings.
IMO, Nokia is doing an extremely good job, not just at marketing, but also at hardware, and apps. At least they were the first manufacturer that put out a Windows Phone that I would want to buy.