^^^^^^^^^^^^^
To be fair Ulquiorra's conflict with Ichigo was also a significant plot point and and the fight between him and Ichigo was one of the most memorable in bleach, just like Grimmjow he was given some development his fascination with the concept of "heart" and his back and forths with both Orihime and Ichigo is proof enough that actual thought was put onto the character.
He is not my favorite character or even in my top ten to be honest ,but then again neither is Grimmjow, the only reason I would prefer Grimmjow to be a part of this war is because his chaotic attitude seems perfect for this arc.
Also I wouldn't count any of the other Espada as more or less cowardly than Grimmjow, they all had their different reasons for following Aizen and at the End of the Day only two had the Balls to actually attack Aizen and even then it was during their last moments. Grimmjow may have disobeyed Aizen a few times but he still feared him like the rest and would get right back in line as soon as Aizen either indirectly threatened him or exerted his spiritual pressure.