The same way you can spend vanguard and crucible marks on world mats (spiritbloom, relic iron, etc...) you should be able to exchange the warlock gear mats for hunter / Titan mats and vice versa. I have over 1000 sapphire wired when I really need Titan plating. It's a huge pain to grind glimmer, chests and drops to get the required 25 per upgrade when I have an excess of shards. I think this would be an easy fix that would only encourage people to build on their alternate classes.