NOTE: I'm not an expert but I consider myself an advanced user when it comes to Android.
The antivirus part of those apps are pretty useless as far as I understand regarding Android and Linux (which Android is a mobile version of). Malware is somewhat more concerning but as long as you use common sense when downloading apps you really shouldn't have a problem. I tend to only use apps that already have a large number of users and have a decent rating, or if they're are recommended by Android Central or Android Police which are pretty knowledgeable Android blogs. The remote locking features of these apps can be really handy if your phone gets stolen so they do have their uses. And unless it's killing your battery, it doesn't hurt to have it.
Also, on a somewhat related note, DO NOT USE task mangers. They actually cause more of a battery drain. The Android OS does a really good job of managing it's resources. Task managers just kill off apps that then restart themselves and end up using more battery than they would if you just let them be.
