First and foremost, 343 did a phenomenal job with the voice lines for the grunts, they are extremely entertaining for players.
Regarding the topic at hand, anytime you clear out a group of enemies and leave a surviving grunt, his default programming is to just run around like a headless chicken for 5 seconds before reverting back to shooting you with his plasma pistol. How he musters up courage to shoot you again, nobody knows.
Instead, 343 should program the surviving grunt to stand still after 5 seconds of running around, then give players the option to press RB to recruit the grunt as an ally, if you walk near him. Maybe the grunt will say some funny lines too once he joins you (as a way of sparing his life). If you refuse to recruit him, the grunt will run away, permanently.
This way, it would be possible to recruit an army of 100 grunts, making tons of jokes along the way, while you approach the Banished outposts on foot. Imagine the brutes’ faces when they see you and a massive swarm of grunts at their doorstep, it would be a really fun addition to the campaign (since you can only recruit marines right now). Imagine all the dialogue possibilities when you have all these grunts and marines together as allies.
By this logic, I also wish 343 gave players more options to recruit a more diverse cast of survivors, other than UNSC Infinity’s marines. Were there no allies from the Arbiter aboard the Infinity? It would be really cool to have elites join you alongside your marines in battle. Likewise, players didn’t have the option of recruiting Craig the Brute (or any other friendly brute) to fight by your side, or any of the fallen mini-bosses such as Chak 'Lok during the campaign. For many of them, it makes sense they would rather commit suicide rather than join you after you defeat them, but at least Chak 'Lok shows respect towards Master Chief.
Any thoughts on grunts joining you during the campaign? Hope you are all having fun with the campaign!