I agree that it is a much better invocation. But, we're not using Dragon Magic in our current campaign as freely available material. The material exists, but the powers it contains must be discovered/aquired in game and cannot be chosen by default. So, warlocks do not have free access to Dragon Magic invocations like Eldritch Glaive.
This concept of "discovering" new material always leaves a logic flaw for classes like the warlock or sorcerer where their abilities aren't "learned" but rather a natural part of their being. Warlocks don't have a "research" method of learning things. Sorcerers sort of do, but all it does is allow a method of acquiring spells - they never gain (shy of taking feats) additional spells above the number listed in the table.
If it was my game - I would have the player tell me what he wanted (from other books) and I would either approve it or not on a case basis. For the warlock the abilities gained probably would start to have some sort of physical effect (appearance wise) - especially if they were heritage feat base things.