This blog will be answered by a tirade of comments, mostly consisting of "no teh grunts r dum", from noobs who didn't get beyond the second paragraph. Now that I have hooked you into reading so that you may prove that you are not a noob, let's begin.
I have noticed on various Halopedia forums, blogs, and talk pages reference to "inconsistencies" in the portrayal of Unggoy intelligence. Books like Halo: The Flood and Halo: Contact Harvest make out that the Unggoy are actually quite smart, able to carry out complex military manoeuvres and other actions, such as their attempt to put a grenade into John-117's suit. Other sources, including the games and the Nylund novels, make out that the Unggoy are unintelligent, animalistic, cowardly, and poor tacticians. The latter interpretation of Unggoy intelligence is the one followed by the majority of fans, which seems to have created the somewhat racist opinion that the entire Unggoy race is stupid and cowardly. I intend to prove that opinion false.
The general opinion is that the Unggoy are stupid, cowardly, and incompetent soldiers, compared to Human fighters, who in contrast appear to be intelligent, courageous, and well-trained. This is understandable, since given all their advanced technology, Humans of the 26th Century will probably have pretty good educations by modern standards. Furthermore, Human soldiers are utterly committed to the defence of Humanity against a genocidal enemy and have every reason to fight well for the preservation of themselves, their homes, and families. All these factors combine to create soldiers that are such good fighters even the Sangheili remark positively on their martial skill.
In contrast, we have the Unggoy, who are a downtrodden, oppressed slave labour race in the lowest tier of the Covenant, working in menial, dangerous jobs. The Covenant leadership, therefore, has no reason to educate them properly, since it would only cost a great deal of time and money to give them knowledge that is of no use in manual, menial tasks, and also increases the possibility of rebellion - intelligent slaves who have no desire to remain slaves will revolt. Furthermore, we do actually get well-educated Unggoy in the form of the Deacons like Dadab, who are presumably snapped up by the Prophets for their early devotion to the Great Journey, and educated as priests to preach to the rest of the Unggoy, who without educations of their own and a miserable, poor existence, will be pretty impressionable to religion.
And now to Unggoy cowardice, well what do you expect? Unggoy soldiers are unwilling conscripts who have no desire to fight in a war in which they will very likely be killed, and there's absolutely nothing in it for them, and will take any opportunity to desert (for example, the death of a leader). This is in contrast to Human soldiers, who fight hard and well because they know that the only other option is annihilation. Furthermore, Contact Harvest told us that if Unggoy are well-motivated, for example, in a bid for independence, they will fight ferociously, to the extent of being able to defeat even the Covenant's superior ground forces.
And now we come to the Unggoy's poor tactical sense and lack of proficiency in combat. Well, as I've said, why would they fight well when they've got no reason or desire to be there, and really don't want to be killed? Furthermore, why would the Covenant want to train them well and give them good weapons when Unggoy breed prodigiously, so they have a bottomless supply of cannon fodder, and well-trained, smart Unggoy slaves will only lead to a rebellion? It's a lot cheaper just to give a thousand Unggoy a plasma pistol each and tell them to charge a bunker or get fed to the Kig-Yar. Chances are at least a few Unggoy will get into the bunker, and if not, there's always a hundred thousand more to pick from.
So, there we have my challenge to the perception of the Unggoy as stupid and cowardly: It has nothing to do with genetics, merely environment and upbringing. Based on what we've observed from Dadab and Yayap, I reckon that with the same education, an Unggoy could be as smart (probably smarter, given the kind of people we get on here) as any Human.