You don't get it. The majority of the gear a raid drops is also useless if you go by that logic because the current raid tier can easily be done in minimum ilvl requirements. It's a stupid argument. People don't just want to have BiS weapons for raid content. Some people simply want to have BiS weapons because they want to get the most out of the character they are using and then others like me simply want it to have that little edge for when SB comes out. Yes, the weapon will be replaced, yes it will most likely only be BiS for the first 5 or so levels but if it gives me a small edge in leveling speed than it is worth it to me.
You seem to assume that everyone treats BiS the same way, but let me assure you that people have different opinions and reasons for why they want certain gear.
The bottom line is that the current system is unfair. The BiS gear should never, at any point in time, be tied to an rng mechanic. It's an unfair system.
