Here in the US, metric cutters are a bit hit-or-miss. Some sizes are not to hard to find, but none are as easy as it should be. Hopefully this will get resolved soon, metric everything seems to be gaining, but we're not even close yet. Just getting 8mm shanks is hard enough.
They have become my preferred sized for many things. I have always been a 1/2" fan, preferring them for all but laminate trimmer sized bits. There is quite a bit of added cost though, so I have been leaning toward the 8mm, at least in the mid-sized bits. Both the OF and MFK can use them.
Since most of the spiral fluted bits are solid carbide, the cut diameter and the shank diameter are usually the same. I don't really know why, as most hand held routers can't use many of them. CNC machines with ER type collets can fit nearly everything. Maybe that the market?
If you find an 8mm spiral cutter with 8mm shank, post it here.