Last updated: June 2018
I've always been a user of non-modular power supplies, primarily because they are the cheapest, and haven't had to deal with modular ones very much...but when I do deal with them, I don't like it, and I don't recommend them to any new system builders. Why?
Because modular power supplies suck!
The first reason why, is because you have to keep all of the stupid modular cables of the power supply. Where do you put 'em, in spare room in your case? Leave 'em connected? Leave 'em in the retail box?
The first two options defeat the point of a modular power supply, while the last option leaves you bound to lose the cables...and you should never, ever lose the cables, because if you do, you're bound to make a terrible mistake, far worse than just buying a modular power supply.
mix your modular power cables up.
They are NOT cross-compatible between power supplies, and are likely to fry your hardware if you mix them up. That link right there is full of peoples' horror stories about losing their hardware to mixing up their cables. It's a very sad situation, and you can avoid it easily, so as long as you don't mix modular power cables...or just avoid modular power supplies altogether.
People say about the CM N200, my favorite computer case, that you can't route the 24 ATX main power wires behind the motherboard. They'd probably also be saying "you better have a modular power supply". I kept my expectations low and my hopes high, and guess what I wound up doing? Lo and behold, I routed the 24 ATX wires behind the motherboard, and used my good ol' non-modular power supply. I had no problem hiding excessive cables in this MicroATX case.
However, one thing to note is that I've also been primarily a user of lower-wattage (<500W) power supplies, because I (and a lot of people) don't need anything more. How about power supplies with higher wattage, and more connectors as a result?
Bigger power supplies mean bigger cases, so what I have to say to this is that really-good ATX cases like the NZXT H440 hide the power supply entirely, and offers a big area to hide power supply cables. This invalidates any arguments about non-modular power supplies being ugly, as you can't even see the power supply, nor its unused cables at all.
Sure, sure, not everyone's gonna buy the NZXT H440. Then what? Even garbage ATX cases these days often have space to hide your unused cables, so good ones definitely will.
Plus, if you buy the correct power supply for your system, you're going to be using most of the cables on it. Why buy a 1000W modular power supply with a bazillion-and-one connectors for a system that only needs a 500W non-modular power supply with a few connectors? "Upgrade room", you say? A system that requires a few connectors doesn't turn into one that needs a bunch overnight, the only valid "upgrade room" situations I can think of would be adding more hard drives or another video card, which doesn't require that many extra connectors, especially these days, with today's efficient video cards.
Besides, spending more on a power supply leaves you less "upgrade room" in your wallet.
If you plan your computer purchases wisely, you won't even need a modular power supply to start with, and thus, you can avoid all of the problems associated with them.
The one and only exception I can think of at this point, is that a modular power supply can last you better on several different systems, if you choose to rebuild your system several times, and always know where you hid your modular cables for the times you'd rebuild. My argument to this is that you shouldn't have to rebuild so many times, much less need such different power requirements each time. Why not just buy a nice so-and-so watt non-modular power supply, and stick with whatever hardware is compatible with it?
But whatever you do, never mix your modular cables up!