>in general there is no advantage to the tripple over the double compact at the low end<
The advantage - or difference - is an extra low gear over a compact double with a similar cassette. Some people need that advantage. Or think they do.
>and the tripple has fewer descrete gears<
As I posted above, this isn't true. Typically, with similar cassettes, the triple will have three extra gears, one higher, on lower, and all more closely spaced together than those found on a compact double.
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