Acceptable varies from player to player, also so does "realism" and the "goal of the game".
.5's are fine, they're designed to be used.
multiple .5's is alright (though the design part is trying to be cleaned up for next age), as it has a good economic motive behind it (again. this is trying to be cleaned up). more then a few might raise a little ire, but -shrug- in my opinion. a few dozen to the same city... thats... see next statement.
Almost nobody likes high frequency, multiple overlapping routes. They're technically not illegal, but of late they're becoming problematic.