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What other decisions are left to a ref's 'opinion'', rather than application of the rules?

So you don't think the 'hurry up' from the ref will be quickly followed by a "go forth and multiply" from the player and probably his team mates, thus descending a night of pool into a full scale argument.

Playing a shot within 60 seconds is hardly tricky and doesn't rely on one person's opinion of whether or not they've taken too long. If it's over 60 seconds, it's too long, simple. The '30 second' call is the equivalent of the 'friendly hurry up' you're suggesting but with the consistency of a stopwatch to determine how long people get.