Oh for sure it's a better way of measuring than time. I've had to toss coils because the juice would seep through the cotton and leak out of the afc, or give awful spitback, long before they tasted burnt out. But even going by juice consumption, there's wattage, airflow, length of draw, nic%, etc to consider. And not everyone is going to remember when they put in a new coil, or how much juice they've put through their coil.
So while I agree it's a better way, I just don't think there can be a really accurate way with all that's involved. We just give pointers and relay experience and hope for the best. And IME, it's rarely the coil itself that's the problem. It's usually the user, but they don't want to hear that. LOL