Subs will make your entire system run better. They will lower the distortion in your mains when using a highpass filter. They will give your system more dynamic range (output). They will most likely add bass extension depending on your speaker. Subs done right are always better than no subs.
not to mention you can place the subs for bass while not effecting the soundstage negatively. I can’t recommend subs enough but and a big but is you need a highpass filter to sound right.