Sucker sign
6:5 blackjack increases the house advantage in single deck play by 1.39%. The typical single deck Strip game with 6:5 blackjack has a house advantage overall of about 1.45%. This is worse than the line bet at craps, and worse than many video poker games. In fact, it is possible to find slot machines that give you better odds.
The typical rationalization used is "well, I don't get a blackjack all that often, so how much can it hurt?" A lot, given the fact that you get a blackjack about once every 21 hands. Giving up 60% of the blackjack bonus is huge.
Every person who sits down at a 6:5 table might as well be wearing a big sign on his back that says, "Sucker." That's what knowledgeable players see when they pass by a 6:5 table, and that's what the casino sees as well.