Having something that will do OBDI is going to get bulky to carry, as different vehicles will need different cables. And you're usually looking at a much higher $$$ setup to do it (OBDI was much less standardized than OBDII). The upside is, a lot of the more $$$ scanners that could do this will also support the more manufacturer specific OBDII stuff, rather than just the generic stuff (which is usually good enough for on-the-road diagnostics).