XL5 why issue conditional clearances, it only leads to ambiguity, which in high density environments leads to possible confusion and misinterpretation. Leaving scope for error and potential disaster, absolute clearances which due to the human element are not perfect, but surely do greatly reduce the margin for error.
The set up in the US can work well on a good day, but has very little allowance for the lowest common denominator. The below average guy on bad day, and through any number of influences we are all capable of being the below average guy.