Fair enough. I think every choice has a price. I kind of doubt his plans are going to roll smoothly, even if you do spare him. Even Odaviing doesn't think he'll succeed with other dovah. At the very least, I expect more conflict with dragons, no matter which side you choose. It could be conflict with dragons as a Blade, or one with Paarth on your side again. Or maybe Paarth loses his mind. You never know. I just don't think it's that obvious. Kind of like in Dragon Age, if you spare Connor. If you've played it, he's sent off to Tevinter in the end. If they ever write that character in again, he'll probably be an evil mage.
I chose a pragmatic approach in my opinion. The way I see it, the blades aren't going anywhere. And as long as Paarthurnax lives, whatever knowledge he may have, that could be used will still be available should the need arise, whether he is successfull at rallying other dragons to his way or not is just a bonus if they help me. And if he betrays me? Well, he isn't exactly remarkable when it comes to combat anyways, and he's already given me a deadly weapon against his own kind.