No need to get zealous over it. It's a throwback to WC3 Dota; She had one there, although it was much more... "rendy". Spectre doesn't even have one anymore, and Vengeful is small enough that it doesn't really stand out on the hero. That being said, you could have said the same thing about giving Skeleton King a shield…
I've read stuff from Valve saying they want contributors to help grow and develop the lore. It's also somewhat of a throwback to the old dota where she was using a WC3 model and had ears. In the lore it says she's not a Drow, but I don't know how many bluish purple humans there are. I think she'd still be an elf or…