Weather radar seems to primarily only exist on twins, with the single exceptions being various 210s (mainly on the P210), Malibu series, a few A36s, and the various turbines out there.
The problem is that if you didn't purchase the plane with some form of radar in it already (or having had some form of radar in it at one point in time), the installation becomes quite difficult. Even when we put the KWX56 in the 310 (which previously had an RDR-130), the install had some challenges due to the small radome on the plane and differences in size between the old unit and new unit. We made it work and are happy with the install, but it certainly had its challenges.
I agree that a stormscope and XM can provide you with good coverage in most conditions. In some conditions, you can have the stormscope be more useful than on-board radar, simply because lightning and areas I don't want to go through can exist without significant precipitation.
Overall, ATC can be helpful at times, but I do not trust them as much as I trust myself. I've had enough instances when they would have happily let me fly through a thunderstorm (or try to make me divert around something that was insignificant) to realize that, while some are very helpful, others aren't.