The main assist function is a great addition to the game. (If it would only get used in PuG situations it'd be awesome.) I love the versatility of the Assist addon. The biggest problem with these functions is the difficulty in switching target callers should your primary caller go down. I call this static assist. The other problem with this is the enemy finds out who the main caller is and takes him down first, you are going to lose quickly unless you can adapt.
My solution to this problem is what I call Dynamic Assist. I can assist any player in my party simply by pressing a hotkey. No, I don't put 5 buttons on my hotbar to do this. That would be wasteful. This is what I do instead.
Under your key mapping options under the WAR Main Menu (the WAR icon in the top middle of your screen by default), you'll see the default options for assisting your party members. By default, these are set to Shift + F2-F6. By default Shift 1-5 rotates your main hotbar. How many people actually make use of that? (My guess is about zero.) So I made Shift 2-6 assist group member.
With my Razer Naga, it's very easy for me to move, target, and fire off abilities all at the same time. Now I can also assist any party member I want easily. I don't get to use this feature as much as I'd like since I usually end up being the target caller in my parties, but when I'm not, I can assist anyone in the blink of an eye without missing a beat. If you don't have one, you should get one. For $80, it's one of the best things I've done to improve my game.