My experience with Lookout on other platforms was that it didn't include antivirus, but had features I wanted. And I've used AVG for years on PCs. So, I put both on my phone. It turns out that there's more redundancy than I expected. But, if PC antivirus databases are any indication, they each miss some that the others catch. And there doesn't seem to be any conflict or performance hit. So, I'm going to leave both on my Note 2.
As for whether A/V apps are needed, I'd rather be safe than sorry. Some say that the variety of Android versions makes exploits hard to create, but there are certain things that have to work the same way across versions. Do we expect all of them to be unattackable? Others suggest that our own habits may leave us open to attacks. I'm very security minded, am careful about social engineering, and careful about where I go on the 'net, but I still had a problem occur on one of my WM phones. So, I'll agree that what you use or whether you use any security apps is still a personal choice. But I'd rather unnecessarily have them on my phone because nothing happens than find out the hard way that I should have used them. My $.02.