In LTE Preferred..... mode your device will first search for LTE transmiter, then HSDPA/EDGE and in the end GSM transmiter.
So you will lose battery for searching anything else than GSM (2g) network.
I use GSM Only network because:
1. baterry drain is less,
2. I dont need so fast internet for readings mails, using Whatsapp or tapatalk. At home I use WiFi or when I really need fast internet - I use free WiFi spots in my town.
3. I'm in signal of 2G network all the time, what can I not say to 3G or LTE.