Every WHM -should- have this piece. It doesn't add resistance points (the ones you can see in your equip menu) it gives your bar spells the added effect of Magic Defense Bonus+5 for magic damage of the appropriate element. That bonus is equivalent to the mag. def. bonus from 2.5 merits in the bar spell category. Contrary to what Taryia said, they do stack with AF2 legs. This is also the best piece for max mp and its very affordable.
The reason that these are only worth 50k despite the high cost to make them is because enough were made pre-patch when galateia weren't worth that much. When galateia jumped in price these didn't because there was a supply of people selling them already, WHMs who don't want it any more, need gil for other things, etc. It's not crafters who are keeping the price low, it's the people who previously bought them.
Looking at the mats it takes to make this why are most people selling these for 50k? People *** about not being able to make money and on a 100 cloth recipe you idiots sell these for 50k.
Good body part for max MP, enmity and barspells, not a must have at all, just a nice addition.
Also, STOP bitching for skillups, if you can't do it by yourself and eat the loss, you don't deserve to reach Clothcraft 100