My 5d has ISO 50, but it's marked as L and you sacrifice dynamic range. For the 30d I'm not sure about 50 as I don't have one but I know for sure that it has ISO 3200 marked as H. My old 1dII also had ISO settings L and H, but my old 350d and 300d did not.
Generally, ISO 50 is present on compacts as the noise control for higher ISOs is not as good as DSLRs, which makes using ISO 50 a necessity for clean images on some compacts, especially older ones. I guess Canon just felt that there was no need for ISO 50 since ISO 100 produces uber clean images on their DSLRs. Some Nikon DSLRs only go as low as 200.
Of course there are reasons to use ISO 50 other than to achieve low noise, but often, this is not the reason why ISO 50 is used on compacts. Anyway, with the decrease in DR, it is preferable, at least for me, to use an ND filter if I need to cut the amount of light in a scene rather than use ISO 50. I do appreciate it being there when I need it though.