I already use the second version of the count function you give (without
default arguments which I am not a big fan of). I'm not saying its
difficult or unreadable to bypass itertools.count and write your own
enhanced count function. But if I use a custom count function, you use a
custom count function, and possibly many others too, then there could be
a common requirement for a step argument and it might be a good idea too
make it more widely available. 

Haskell has a powerful and concise list notation, some of which Python
has already borrowed from:
multiple3 = [3,6..]