Ok, if it's gameplay reasons thats fine, but don't come with realism.
It is absolutely not realistic that future fusion bombs will rely on radioactive material, a Hydrogen bomb has a lot more power than there is uranium required to start the fusion, and the bigger it gets, the less you need relatively.
With the technology depicted in this game, pure fusion should be possible at higher tech levels, and thus radiation per strength should go down.
As for bombarding planets for fun and profic, guess what, now you need a terraformer in orbit, add a few toxic gases (then again, how do we even know they are tocix for that race?) and wait a month.
And if you don't have time for that because an enemy fleet is approaching, the radiation does actually benefit you.
Theres enhanced Radiation Warheads for that after all. I don't actually see the benefit in killing a population when it's a lot faster to build up industry if you just overtake everything on the planet, even without radiation. Aside, waiting two years is not a long time in this game.
So, yeah, I see no problem with neither realism nor gameplay and always use SM functions to get rid of radiation in my game, and I've yet to run into any problems of logic of gameplay.
Still, the argument about ruins is a valid one, so lets just forget it again.
Discussions about radiation never end well, I guess. (See the Orbital combat suggestion, that didn't receive too much attention).