Wrynn probably isn't stupid. If he were, that would be the easiest way to explain his irrational actions. I find this to be very unlikely though.

Could he be getting bad information? This is more likely. Yogg-Saron seems to be implied as being, not only "a" death god, but a master manipulator. After all Yogg corrupted Deathwing, and very well could be causing all the problems which are distracting the 5 Dragonflights. Manipulating a mere human? Cake walk for an Old God. Although never mentioned ingame, Onyxia was influencing the events in the comics when Wrynn was in the Arenas. We all know the Black Dragonflight is basically an arm of Yogg-Saron.

What is a tad confusing is that, as mentioned ingame, only 4 years passed from the end of WC3:FT to WoW. So Wrynn would of been well aware of the fact that the Horde was crucial to the success vs the Legion. He had just been kidnapped when WoW launched. Supposedly he was going to meet with Thrall at Theramore for a summit.

Simple version is I think Blizzard just wanted a prominent antagonist. Lich King and Malygos are the Big Baddies but are clear-cut "bad". Wrynn can potentially do a LOT more damage but with "good" intentions.