People recommend Dragon Scale for 42 based on its status resists rather than its split damage. The fact that the split damage is appropriate to the area is rather nice, but it's the status that really makes the difference. Fallen is rather good there, too, due to getting the same two status resists, even though it doesn't get Piercing OR Elemental defense.

There are a few sets of split-damage armor that have no Normal defense. Radiant Silvermail and Dragon Scale are good in niche areas, but aren't really worth getting unless you want to improve your performance in those areas (i.e. they are not all-around useful). The Divine set, on the other hand, is actually pretty good as it resists fire and shock, targets the two damage types that aren't Piercing (and most people aren't Pierce def fans), and the helmet gives a Fiend damage bonus.

But even so, the lack of Normal on these is kind of a big deal, even in Tier 3, and yes there are very few places where you have to contend with two prevalent damage types at the same time. Notably, FSC has elemental damage from fire traps and the enemies mostly deal shadow, so Divine with its Fire resist and Fiend bonus for the Trojans is good there. But if you're short on cash and looking to maximize all-around usefulness from an investment, look to just getting a set with Shadow defense and a set with Elemental defense, and maybe pick up a Piercing set later if you like walking on spikes as much as I do.

As for whether or not mixing two pieces gives comparable defenses, the answer is pretty much no. A full set of Dragon Scale gives you above-average Piercing and Elemental defenses on BOTH pieces. A mixed set of say Vog and Skolver gives you one piece with average Piercing and one piece with average Elemental. The Dragon Scale will be much better against sources of pure typed damage (jelly spikes, retrode beams), and may actually be better against split damage too; based on my experiences with damage tests in the past, I believe mixing sets of armor to be a bad idea and find it's best to wear sets or pieces with the same defense types due to the fact that splitting tends to give lackluster defense against both types rather than good defense against one. But the lack of Normal will make a full Dragon Scale set notably inferior to mixed Normal/Elemental damage relative to wearing full Vog, and Vog works fine against pure Elemental.

With UVs on the defense types in question, splitting armor becomes more viable defensively, but I wouldn't do it without them.

Good point. So Dragon Scale is not perfect; it's merely almost perfect. :)