One of my biggest pet peeves is a pipe that won't pass a pipe cleaner with ease. I will not own a pipe that doesn't. I rarely have to use a pipe cleaner during a smoke to soak up excess moisture. My artisan pipes are made so well they rarely ever need one. There are times when a piece of tobacco will get stuck and if I cannot get a cleaner into the bowl to fix it, then I have to risk breaking the pipe down mid smoke which I will not do. I don't care if the pipe is full bent, it better pass a cleaner.
If I am buying a pipe one of the first questions I will ask if it passes a cleaner.
Most of my pipes are Savinellis (straight and bent) and all of them pass a cleaner easily. Also my Winsløw, Brebbia and even my basket pipes. Only my Peterson Sherlock Holmes series Watson doesn't pass a cleaner - not surprising, a full bent pipe. And guess what; it gurgles as hell…
It is absolutely possible for a pipe to smoke well and not pass a cleaner. It is also possible for a pipe to pass a cleaner and not smoke well. So it's neither necessary nor sufficient in terms of smoking.
But it's sure handy sometimes. Sometimes you need to dislodge a piece of tobacco mid-bowl, whatever. So I think it's important that pipes do pass a cleaner, and everything I make and almost everything I own at this point does so.
Pipe manufacturers are lazy about this stuff - they have equipment which dates from 100 years ago in terms of design, and lots of these ideas just weren't part of the process until very recently. Even Dunhill, where the mandate was to build perfect pipes.... the bents often don't pass a cleaner.
If you are building pipes to tight tolerances, passing a cleaner should be one of your goals as a maker. Is it a big deal to all smokers? No, seemingly not. But big enough to enough guys....
I use pipe cleaners mid smoke much of the time. I will trash a pipe that can't pass one, unless it might be valuable to someone else.
I had to significantly file a Caminetto I bought on this forum to make it pass one, but I'm quite fond of it now. Like Phila said, it is not that hard to get right.
I favor straight pipes anyhow.