I like the idea of grouping by scale.... though there are some things where a "range" of scales is more appropriate, and others where it's not really close enough to be the same. For example:
1/72 & 1/87 are not close enough to fit in the same category. They need to be separate.
1/144 should be a separate category, but 1/148, 1/150, and 1/160 are all regarded as "N Gauge" (They all run on the same 9mm gauge track, even though the scales are different), and could be grouped together. Likewise, I'd expect to find 1/285 and 1/300 (common wargaming scales, often used interchangeably) together.
If someone wanted to put a single model in two categories, they could, just as they can now. For some things it wouldn't be appropriate, others... like trees, haystacks, and some machinery, could pass in a couple of scales.