When the 8x +/-R drives came out with 2.4x on DL media, and the first of the 16x +R drives did 4x on DL (though by uprating 2.4x media to 4x), it was natural to assume that 16x +R support would be paired with 4x on DL by any other makers moving up to this level.
After all, doing twice a normal DVD size, at a leisurely 2.4x, on media that costs more for a single than a 10 pack of single layer, is hardly exciting.
By the time speeds of DL drives and prices of DL media hit acceptable levels, it'll probably be superceded by something better, mind you, unlike the bad old days of CD-R, at least the price of drives isn't a total fright, even though the price of the DL media is.
So far, every drive from my first CD-ROM, through faster, through a slow CD-R, a faster CD-R, and a DVD+/-R, has cost less than the previous one, and I didn't start right at the beginning.