Sir Winston Churchill once said, "However beautiful the strategy, you should occasionally look at the results." Now, you seem smitten with the idea of DR applying to magic spells as a means of niche protection for the fighter by nerfing blasting from a theoretical point of view, but I suggest you consider the actual effects on gameplay and run some numbers. I find that decisions made on facts and data work out better in the long run.
I disagree, and here's why: My view is that mages can set up all these conditional effects and deal more damage than fighters, usually at range. I'd like to at least give the fighters the damage niche.
Ah, so the fact that mages have very powerful, potentially broken, and sometimes flat-out broken, spells is not the problem - the problem is when they start to encroach on someone else's niche. The real issue is making sure each party member has a role in the party which no one can take away from them.
Much like modern businesses, then, where both CEOs and janitors are crucial to the success of a company, though, if I may say so, one of these positions is obviously more important than the other. Whereas a CEO may take a month or two of vacation with no ill effects for the company, when the janitor takes even one day off, things quickly fall apart.
A fighter running around with various weapon types can handle various Drs. That's where this post is coming from.
From a practical perspective, he won't be. At higher levels multiple enchanted weapons are expensive as heck. This is one of the many problems with Two Weapon Fighting styles, and I'd expect having more than two weapons to exacerbate the problem. You sink a lot of money into multiple weapons and are forced to choose between having them be powerful or affordable.