While there are technical advantages to supporting g, as already mentioned, I suspect the real reason Nokia included b/g, despite the inability to realize full wireless speeds, probably boils down to two things:

1) Marketing: "b/g" is a checkbox/buzzword/whatever you want to call it. Saying you only support "b" would look bad.

2) I suspect that the OEM wireless chipset manufacturers have all switched to b/g chips, so getting a b-only chipset is now probably not cost effective, if even possible.

These two reasons combined are also probably why all the new cellphones that have WiFi are also b/g devices, even though they have even less capability to saturate a wireless link than the tablet has.