I'm well aware that it's not by accident. It just doesn't match the flavour presented. I don't think that it's Wrong or even Bad. In fact, I agree, it's good that they aren't all radiant resistant.
I thought the arguments about which damage keywords were best and worst had more to do with the sheer number of creatures that were resistant or immune to those keywords? A quick compendium search backs that theory up (without giving any credence whatsoever to the relative rarity of the creatures in question).
It's all very well and good to say, "poison damage sux becuz three times more creatures resist or are immune to it than any other damage type! OMG! Frostcheez FTW!"
Except if (and I'm just using this as an example) most of them are monsters that most DMs never use, then it doesn't matter. I don't actually know if this is the case in any of the 'damage type x sux' arguments or not, it's just possible that this is the case.
Anecdotally (I am well aware that anecdote =/= fact), I have been playing a necromancer-flavoured wizard for quite some time (now level 9) and I haven't felt particularly "gimped" by my choice to (with DM's permission, of course), re-keyword many of my powers. I guess YMMV*.
* the Y here refers to You in the general, Player's Handbook sense; anyone reading, not necessarily you, Destil.