If you are going to build a PC now, not getting an SSD is the worst decision you can make.
That's some strong hyperbole here.
I'm of the opinion that if you're on a budget and that $100 you'd spend on a tiny SSD for the OS is going to impact the rest of the build, then you skip it. The money is much better spent on a good CPU/GPU. I'd even rank a quality case over a SSD.
It is really easy to throw a SSD in your system later. You can't incrementally upgrade a CPU or a GPU.
A SSD is nice. Windows boots fast, it's a little snappier. But when it comes to gaming, it's such a minimal impact.
I just added a SSD to my system a few months ago. It really didn't change much. Programs open a little quicker. Windows boots faster yes, but I would sleep my system anyways. No real change there.
I know it's not the popular opinion in here so I don't spout it often but when we're making claims like "it's the worst decision ever", I have to chime in.
tldr; when you are on a budget, SSD should be a pretty low priority. Only spend the money if you have it. But get one for the OS if you can.