[discussions/choices]Some of them are between serial and parallel I/O for a number of reasons. Parallel I/O is normally quicker and simpler. Serial I/O uses fewer wires. Where the wires are long, for example, fewer can make a large economic difference. Serial I/O requires more complexity, including timing and protocol. Parallel protocols often require handshaking, while serial ones require clock and timing constraints.
Using a modern chip with the appropiate serial hardware (IIC, SPI, ...) serial does not "requires more complexity"

forgetting the "methods of yore" and instead discussing using a modern uC where you have ALL ports available (no need for external memory) you will have port pins galore so usually there is no problem.

now what if you need more I/O?
Well, if you choose to expand by IIC or SPI you will only lose 2 or 3 pins and still have plenty of fast I/O and, in all cases I can visualize, the I/O (in excess of 24) that 'goes serial' will be more than fast enough. That way there is no need for manhandling an older device into the design. e.g. Richards example of 96 relays would not be 'too slow' if done by IIC and e.g. 3 PCA9698 which would give 120 additional outputs while still preserving ~24 full speed outputs.

now what if you need more data memory (8k is not enough)
a) you could go for one of the ST offerings with more data memory, but that would limit the availability of peripherals to what is avaialble in that series.
b) the SIMPLE solution would be to go for an 8 porter (more and more are popping up) and use the methods described above.

There are lots of cheap "aids" to facilitate serial protocols. The many queries we get about USB, IIC, SPI, ISP, etc, all suggest that they're not quite as simple as parallel I/O.
Again ancient technology even the lowly $1 NXP LPCs have hardware IIC. For the devices that have HW IIC there is 'standard code' that is problem free.

I don't think it's hard to conceive that parallel interfacing of an HD44780-interfaced LCD is easier in parallel than in serial mode
HUH? with my last product I chose a serial VFD because it is simpler.

The decision has to be made on factors other than, "does it work?" because either method can be made to work. Circumstances determine the suitability of each.
Hang on to something Richard, I agree with you :)

a good example of "can be made to work" would be bit-banged IIC when a plethora of devices with HW IIC exist.

Erik