Except that you're not actually *doing* much, since they're very passive enemies. Bungie was trying to allow Flood levels to have some sort of "paced" midrange combat by forcing the Flood to be timid until they're up close and hit hard, but the slow drip that occurs when this mechanism is actually used isn't particularly inspiring. And when the Flood actually close in, you get something that resembles a clumsier and melee-focused version of Halo 1's glorious dancing.
In general, Halo 3's Flood seem sort of over-designed. Bungie succeeded in creating a variety of encounter types, but it tends to not really nail it.
I don't dislike it per se, but Flood gameplay isn't really the highlight of Halo 3, and Halo 1's Flood gameplay is more elegant and exciting.