As a person who has worked in IT security for quite a while, I will add this:
Most AV applications work by:
1. monitoring current activity on the device (file downloads, application install, access to system, permission rights requested, etc)
2. scanning existing files and processes for potential threats
In theory, that should keep you protected, however the reality of it is that in order an AV application to detect a threat, it first has to be able to recognize it. Most AV software will update it's definitions regularly, but the attackers will always be one step ahead, and new threads will often get right past AV detection. The other thing to be aware of is that in many cases, even if the AV engine does detect a threat, there's a good chance damage has already been done. While AV software will attempt to delete and/or quarantine infected files, well written code often find ways to protect itself from common AV removal methods. When asked, I generally tell my friends, "Anti-virus software does a great job of letting you know you already have a virus -- but that doesn't always mean it is able to get rid of it".
My point -- if you want to install AV software, then fine, I don't know if it's necessary or not (I don't use it on Android myself), but if it's not hurting anything then why not? Even if all it is good for is detection, that is still valuable information to have, should you actually encounter malware. The more important thing is though that using AV should NEVER be an excuse to not remain vigilant about what gets downloaded to or installed on your device. When installing apps, stick with apps from well-know developers, check reviews and permissions requests, and finally, just use common sense -- anything that seems too good to be true (i.e. "this app will print free money for you! try it now!") probably is.
Not trying to say the quoted poster above was wrong about anything (except maybe the root comment), but just wanted to add my own input.
For the sake of this post, the term "device" could be substituted for "phone", "tablet", "PC", "Mac", etc... any personal computing device.