This doesn't really work for 2 reasons: first, the weapons that currently provide resistance provide it as a free bonus so the itemization is obviously seen as entirely dismissable by the dev team and, second, melding resistances onto crafted gear comes at the direct cost of other stats, which are the only advantage to crafted gear (pentamelding gives you a metric crapton of stats compared to a "normal" i70).
If the devs really wanted us to find resistances useful, they'd put it on gear that didn't have to be crafted. Either that, or they should allow existing blue gear to get melded so that you can add resistances if you so choose (restrict it to 2 slots max for balance purposes) so that you can get resistance on everything you're wearing (so that you can tailor your gear to the damage type of the content you want to focus upon) but the fact that you can get a lot *more* resistance from crafted gear still provides an explicit benefit.
The problem with elemental resistances/weaknesses/attacks in so very many games is that, because they're so specific, they're useless itemization in all but a minority of content. Even if it's amazingly useful, unless they *require* a certain amount of a given resistance (which WoW did in Vanilla), there's no real reason to use it because it only applies to a small amount of content (would you really want gear that's basically i100 for Ifrit but i60 for everything else?). If they try to provide global resistances so that resistance stacking is actually useful, there's no point in having specific resistances otherwise. The only real way you make it useful but not crippling/annoying (gearing up for fire resist was a friggin' *chore* that was worse than the already onerous attunement quests) is by having it exist as an non-required bonus with a reasonable opportunity cost, such that it's a benefit that is noticeable but not required for those people willing to spend a bit more.