Yeah, man, they know that "dragon blood" means dragon ancestry, and their blood is no more draconic than their skin or their spleen. However, blood is powerfully symbolic and generally cool thematically. Besides, Con damage could represent a strain on the very fibre of the character's being - all of which is imbued with a draconic heritage.

And hey, if you would never trade Con burn/Con damage to avoid using expensive material components, and think it's a weak trade-off, that just means the feat could be underpowered. This isn't a bad thing - when you throw in a new ability, better to err on the side of underpowered than overpowered. I can see it being really useful if your character hasn't got access to the material components, which could certainly be the case if you're out adventuring and need to use a spell you didn't forsee.