btw anyone who makes glowmaps with more than 8 bit is a lame, and you don't need more than 256 colours for shinemaps too.
textures are 256 colors, and the only situations where you really need more color depth are on nebulas, planets and stuff like this, with lots of colors and lots of graduation of colors. But even if it may be reasonable to go for 24bit textures, using more than 8 bits for glows and shinemaps is just a waste imo.
And this is my main concern of using alpha on shinemaps.
unless it is possible to have an alpha channel on a 16bit (8 for colors + 8 for alpha) image, which I don't know (and in this case I'll shut up and you really should go for this solution), you'll be forced to use 32bit (24 for colors and 8 for alpha) shinemaps, which sound to me like a waste.
I'd be curious to know, at this point, if adding a third 8bit image (8bit glowmap+8bit shinemap+8bit envmap) would be more performance hit than having only 2 images (8bit glowmap+ 32bit shinemap) but with one of them 32 bit (a total of 40 instead of 24 bits).
In other words, my question is if it is better to have each effect (glow, shine, env, and in future bumpmap) on an individual file with the lower number of bits possible, than having more effects on a single file with a number of bit higher that what it'd be needed.