Probably it takes a lot of experience to use both correctly. If we are talking about a small programs, no threads, no chances of reuse (no modularity) - in other words "Keep It Simple and Stupid" - then it is fine.

But KISS is difficult to achieve: there's Hubris that pushes you to do "powerful" things rather than getting the job done, there's "anticipatis" that pushes you to have an answer ready for all future changes instead of solving the problem you actually have now, there's deadlines, and there's invasions of external unwanted complexities (silly requirements, interfacing with buggy software/hardware...).

That's why generally speaking "don't" is the safe piece of advice. But those who think they have the basics down can try it (in a harmless context like personal tools) and see what happens for themselves.