I have read that by applying the rail lube, in addition to cutting down on the friction, each time you cock the bow, as the string comes up the rail it carries a small amount of the lube up and into the cocking mechanism; thus, helping to keep it lubricated. Makes sense to me.
While I haven't had any problems with string wear, I have had the serving (where the arrow actually nocks against the string) replaced twice in just over a year. I'm told by my dealer that this is normal wear-and-tear since this is where the cocking/locking hook catches the string.
Another point I would mention (based on my own experience last season)...........be especially careful when shooting your crossbow when near of beside an object. I was in my tree-stand when a nice little buck approached from my rear area........my stand is located inside three large oak trees growing very close together (maybe from the same root base). In any case, as I positioned myself for a shot, I neglected to take into consideration that the limbs of my crossbow would be moving forward as I shot. Naturally, when I pulled the trigger, the left limb of my crossbow hit one of the oak trees and who knows where that arrow went. At first I was afraid that I may have broken the limb on my bow......but after carefully checking it over and later shooting it, I concluded that no damage had been done. Couldn't believe that I had been so stupid.........by not considering the this was not a gun I was shooting, but a horizontal bow.....with limbs sticking out on either side. Doubt that I'll make that mistake again, but I mention it here as a word of caution............
It isn't what happens to us, it is how we deal with it, that matters most.