Is implicit and explicit memory a useful distinction?
No, the distinction is real, not arbitrary. Implicit and explicit memory show different hallmark behavioral characteristics. A good overview of the difference between explicit and implicit memory is available in the MIT Encyclopedia of the Cognitive Sciences. The wikipedia page on implicit memory also has some useful information.
Perhaps the most important piece of evidence we have for this distinction is not behavioral, but from a double dissociation in neuropsychology: some brain damaged patients have impaired implicit memory but intact explicit memory, whereas others have impaired explicit memory but intact implicit memory (e.g. Gabrieli et al, 1995). This is possible because implicit and explicit memory are thought to have separate neural substrates (e.g. Squire, 1992).
There are a lot of questions in your second paragraph and I'm not quite sure I follow. But to be clear, implicit/explicit memory is not the only distinction one can make on memory systems. It is one of many.

Gabrieli, J. D., Fleischman, D. A., Keane, M. M., Reminger, S. L., &amp; Morrell, F. (1995). Double dissociation between memory systems underlying explicit and implicit memory in the human brain. Psychological Science, 6(2), 76-82.
Squire, L. R. (1992). Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans. Psychological review, 99(2), 195.
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