The first and most awesome reason to keep Ellis around is his gigantic balls of steel. This is clearly demonstrated when Ellis bestowed upon us the gem of wisdom that is his motto of, "Kill all sons of bitches." That motto is one so profound it could inspire even the most cowardly companion to turn into a regular wrecking machine of death and destruction.
"Ellis has shown me the way. Uuuuuuuuuh"
Next, is his sense of humor. Nothing is more important in a situation as dire as the zombie apocalypse as someone who can tell a good joke. Think about it. You're tired, hungry, scared shitless, and all you wanna do is go home and stumble on your computer. (Yeah I know who our primary audience is.) Then along comes some insane or wild comment from Ellis' mouth, a comment that is so absurd that you can't help but laugh at. Therefore, allowing you to relax a moment, allowing your mind to rest. This mental rest Ellis provides could be what keeps you alive when you should have already been devoured by your neighbors.
Also, let's not forget the obvious trait as to why Ellis would make a great companion. He's white. Or at least not black. If there's anything we've learned from multiple horror films, the black dude (or lady, don't wanna be sexist here) is the first to go. So you at least know that Ellis won't be among the first to die. Although I'm pretty sure he's Teddy Roosevelt incarnate, but only 10% as awesome. Teddy Roosevelt probably wouldn't even need a gun in the Zombie Apocalypse.