This is because if these moves were to attack first, the oppponent would be unable to attack. For example, if these moves were to be used on a Serperior if they had a priority of +0, people could give Serperior max Speed EVS, a speed boosting nature, and a Choice Scarf, and your foe would only be able to attack if they had an even faster Pokemon have those same things, or if they had a positive priority move. Or if they had a Pokemon like Octillery. And if you were to combine +0 prioirty Dragon Tail Serperior with entry hazards... So yeah, it would be way to hard to counter, and considering the number of Pokemon it is available to, the whole move would have to be made Uber if it was +0 priority. Although, here's an interesting tidbit, you can use Restalk, as when Dragon Tail/Circle Throw are used with Sleep Talk, they have a +0 priority.