By the time you get tier 5 +a triple the minions that are offered at tier 6 are not strong enough to be worth dropping in your line up. They should be stronger or have buff battlecries, like the mega don that adapts all your murlocs. He is a perfect example of what a tier 6 minion should look like imho. In fact he is better than pretty much any tier 6 minion you can get.
Just a suggestion, it can be difficult to get a tier 6 minion so I just feel that they should be big gamechangers