truth be told, Paarthurnax does not give you dragonrend, the shout that was used to defeat Alduin the first time, that was created by humans specifically for that purpose and was lost to time. he only told you about it and the college pointed you in the right direction. he does not give you the elder scroll either, in fact, he explicitly says that he has no clue where it is.
If not for Paarthurnax, you would not know how to learn Dragonrend. He told you about the Time Wound. He told you that you could use an Elder Scroll to look back through the Time Wound to learn Dragonrend from its creators.
Furthermore, he helps you defeat Alduin at the Throat of the World, and resolves to teach the Way of the Voice to other dragons. Paarthurnax is a far better influence on the world than the two idiot Blades that remain.
Edit:
If the Blades tell you about Dragonrend, how to learn it, the Time Wound etc. I did not see that in game. If this is true, then some of my above comments are wrong. Still, the Blades are genocidal. Where Paarthurnax would teach dragons self control, the Blades would simply murder them all, simply for what they are.