I'm sure something similar has been posted here before, but I really think this needs to be addressed.
Anyway...so the basic idea is item level should increase with materia melds. Obviously there should be some restrictions, caps, and math to it, but over all that's the general idea. I know with 2.2 coming out there will be a new gear tier and this won't matter as much for current content, but if there are new say...item level 80 crafted items, then this will be very relevant.
Let's say you take a ring like Gryphonskin Ring. That is better than DL STR and DEX rings pretty much from the start. Once you slot it it becomes much better. Especially for tanks. Since there are no item level 71+ accessories until Allagan/Hero's tier I think allowing for materia to increase the item level for the accessory makes sense.
Reddit thread with support:
Here