[smackfu]

Well, the real reason is that the existing subscripts are to support UPA: http://en.wikipedia.org/wiki/Uralic_Phonetic_Alphabet

Here's where they argued that they needed subscript versions, rather than using the existing greek letters with a modifier: http://std.dkuug.dk/jtc1/sc2/wg2/docs/n2442.pdf

It is kind of a mess though. Maybe it would have been better if they had named the repurposed glyphs like LATERAL POSTVELAR CONSONANT, but that's not generally how Unicode does things.