Lydia will sneak around with you sometimes, but she's seen so much easier which makes her attack.

Also Beowulf (Far more epic than anyone) travels with companions. A lonely quest is pointless if no one can see your awesome adventures unfold.

Also Lydia can die everywhere anytime. The only way she can't die is by Traps, I've tried and having her as a follower for more than 110+ hours. If your follower falls behind he or she has a very slim chance of dying, I myself haven't seen it yet even though I've ditched many, many battles from Giants up to 2 ancient Dragons and Lydia stayed and numerous times have I thought she died, I would wait and hour than another hour, than I was like "fuck it, I'm going" So I left and witout notice she was behind me, I'm guessing she either escaped just nearly or killed the enemy or just simply teleported to me. The only follower that doesn't die is Barbas the stupid fucking dog.

Also it levels up Alteration and Restoration a lot if you keep Healing Other and using Courage on her.

I think the reason why some of you guys haven't seen Lydia die because enemies sometimes stop attacking her when she goes on her knee and aim for you. Also when you're in cover from an arch mage and Lydia is going full force at the guy with bad ass Ice Spikes she WILL die. to be honest I haven't seen Lydia get killed by a dragon, maybe 2 would do the trick because she always heals when they dragon flies off after attacking.