Most projects around here are using other microcontrollers in the same product line from AVR - but those usually have dedicated USB hardware built-in and often with more program memory.
There is freely available program code for doing USB anyway on AVR 8-bit microcontrollers without USB hardware, but I don't think that you would be able to squeeze that and anything useful into the ATMEGA38PA's because it has only 4KB of program memory. A program (called "firmware" on microcontrollers) for controlling a keyboard could be 4KB large without the extra USB code or any special features.
It could theoretically be used as a "slave" unit in a keyboard with multiple parts, where a more capable microcontroller in another part would do most of the work.
The microcontroller will also need support circuitry for producing a clock signal, etc. and you would need some dedicated hardware for programming the chip.
You would not need crazy skills in electronics or anything, but it would not be as easy as with any of the ready-made controller boards with USB that are used in most keyboard projects. You might need to do some programming though.
In conclusion, I would not use the word "garbage", but I don't see them being useful.
