when things are exposed to userspace, the bock based api is needed more

When userspace attempts to bit-bang multiple lines at the same time, you have the added problem that the userspace app may be preempted (potentially for a significant period of time), and this makes reliable operation much harder.

It also means that the reliability degrades significantly as the load goes up. This will mean that it may never fail when the developer is testing it, but in teh real-world with higher load (and much longer operational periods) failures are almost guaranteed to take place.

If you try to group together GPIO lines that are on different hardware, you aren't going to be perfectly reliable, but in most cases, the GPIO lines really are grouped on the hardware level, non-uncommonly, to the point where some layer is reading the state of a bank of lines, modifying it based on what's requested, and writing the new state out to all the lines at once.

P.S. has anyone made a shim layer to allow you to use a parallel port as a set of GPIO lines?