Yes, à priori it does not make any sense.
Now by looking a bit closer one can realize this type of conversion is not exactly useless: it homogenises all the damage sources. Now your Firestrike deals as much cold or lightning damage as fire. Why do I talk about that? No idea man, just showerthoughts.
Play cenos zenith mod, the elementalist can do this.
I’m not too much into modded stuff. Also it looks like the mod is from 1.5 years ago, no idea if it still works.
It was updated less than a month ago. 
Yes, it still works. Specifically, a mastery in the mod offers three passives which grant up to 100% Elemental -> Fire, Cold, and Lightning. The passives are not exclusive, so taking all three gives you 100% Elemental -> Elemental (because of the way Conversion adds up, you can actually get away with having 66% Elemental to Elemental three times over and have the same effect, saving on skill points. ^^ )
My bad, I guess I was on the wrong page. Looks like there are several.
This is a pretty interesting concept honestly!
For me the idea came recently, when 1.0.1.0 gave Speaker for the Dead 50% vitality converted to cold. If we get another item in the expansion granting 50% cold converted to vitality, this is a great way to have all pure abilities become 50/50 hybrids. That could be interesting against resistant enemies. 
Yo dawg…