I don't know why anyone would outright call them "undesireable". The 'C' model outsold the 'K' and 'F' models combined, 2-1. Perhaps the huge popularity oft hem back in the day and the resultant market glut make the base and sport models appear "rare" 35 years later.
The Custom has different geometry than its siblings (actually they each have their own tweaks to the front end), set up to be more comfortable over longer trips. This unfortunately results in far less sporty handling than the Super Sport CB750F. To paraphrase Tony Foale, "everything is a compromise. The fastest, most razor sharp racing bikes ever would be miserable on a long trip; the rider would be cramped and aching in places he didn't know he had, and his teeth would eventually be rattled out of his head. Conversely, the most comfortable touring bike would suffer greatly in a no-holds-barred road race."
The 750C is a sweet bike, but may not be the first choice for making into a performance oriented speed machine. Maybe better stock if you wanted to build a cb750 bobber or brat. The F is more geared towards the performance-minded (and not nearly as pleasant to take on trips, I have one of each). And the K could go either way, as I'd assume it's the middle-of-the-road setup.
