Should You Go for a Modular PSU?
PC building has made leaps in terms of how good the current hardware is and that can be seen through not only the pure performance specs but also because of added commodities and features that make building a PC a lot easier.
One of such commodities are the modular power supplies which make your life a lot easier in terms of cable management. These PSUs allow you to keep your build cleaner and organize the cables for your other parts in such ways that when you need to swap or fix something the cables will not get in your way.
It is pretty clear that modular power supplies are a neat feature that everyone could make use of but should you spend the extra money on a modular power supply for your current (or future) builds or not?
Budget
The most obvious answer to the question of whether you really need that modular PSU is if you can afford your entire build as planned with the extra modularity feature of your PSU. The important distinction here is that unlike your wattage or your energy efficiency rating on your PSU, modularity does not add much in terms of raw utility to your build.
This is an extra feature and should be treated as such if you are on a tighter budget and need to get as much juice for your components as possible. If you plan on overclocking (CPU/GPU) your parts and are looking at a 650W 80 Plus Bronze modular PSU and an 850W 80 Plus Gold with no modularity then your choice should 100% be the higher capacity and better energy efficiency part.
No matter how much neater your build will be without the extra hanging cables, if you can not output enough wattage to power your components your build is toast.
Going for a Compromise
A much better solution to your dilemma between going with a modular or non-modular PSU is to go for a compromise and buy a semi-modular PSU. The main reason for this is that these units are a bit cheaper than fully modular power supplies and still offer the necessary modularity without being as clunky as fully non-modular units.
Non-modular PSU source: Corsair
Semi-modular PSUs, as explained in this article, use an integrated 24-pin motherboard/CPU cable but the rest of the connections are modular. There is no way around the necessity to use the 24-pin cable/CPU power cable so even on a fully modular PSU you will need to use those.
Semi-modular PSU source: Corsair
What this means is that a semi-modular PSU will give you all the modularity you will need for the other components like SATA power cables and GPU cables while having only one integrated cable that you will need to use anyways. There is really no downside to this trade and unless you plan on using custom cables for your PSU you are getting a great deal with this compromise in your PSU choice.
Going Modular
If on the other hand your budget is big enough to fit a modular PSU and you do want custom cables to fully modify your build to your tastes then nothing should really convince you otherwise. Going fully modular also applies when you know you will have a bunch of drives, RGB hubs, power-hungry GPU, etc.
With that many devices that need extra power cables, you might as well go modular and route them properly to save yourself from headaches when you will need to troubleshoot them down the road.
Fully-modular PSU source: Corsair
A lot of the higher wattage power supply options are fully modular anyways so if you need 1000+W for your build you will end up with a modular PSU anyways.
Conclusion
If you are on a budget and need the best possible performance out of your build, always prioritize wattage and energy efficiency over features like modularity.
If you do want some modularity but are still trying to get the best bang for your buck then going semi-modular is a smart choice since the integrated cables will have to be used anyways, modular or not.
If you have a decent budget or if you are building a powerful machine then going fully modular is the best choice that will most likely happen naturally since PSUs that output the necessary wattage for those kinds of builds already have such features.