Hmmmm I'm kinda on the fence about alts joining farm party's when said alt hasn't cleared here's why.
1. Its an alt it will most likely not be as strong as their main.
2. Their main and alt may have play diffrent roles jobs. Sure they may both be dps but a BLM plays much differently than say a BLM.
3. Even if a person say their an alt but they cleared it on their main. How do you know if it's true from what I understand there's no way to even see if people are telling the truth like this.
I will agree that something like this may be needed but what OP is suggesting may be too extreme for SE to even consider.
Here's my story of what happened. Me and a friend wer trying to help our other friend get their clear in T12 in 2.5. What ever can't remember when lock out was removed. Any way I made a clear pt in PF we got in wiped a lot our friend decided that they weren't going to waste any more of our time so I dissbanded the party because me and my friend wer going to farm T12 since we had our clears. We filled fast once we got in bonus message came up (I will let it be said that I think the visible bonus message is one of the stupidest things ever.) Once we saw that message me and my freind wer like nope not happening when our freind still needed their clear wer not putting up with people who join a farm with out telling before hand that they haven't cleared. Majority of the party left because one person was inconsiderate. And guess what.
It was a person from our earlier clear party....... Thing is we found this insulting when we were trying to help our freind and some others to clear it only for this to happen.
