"Important" is all relative. IMHO, for a straight, clean bass tone - like modern gospel, pop, funk, etc. - a cab sim isn't necessary at all. Unless you want it to sound like you're actually playing through a certain type of bass rig, as far as the various EQ curves and feels that different bass cabs can give you. But I prefer a "warmer", "fatter," more "vintage" tone (while acknowledging all of those terms to be relative and difficult to define/quantify). I typically play with a slight bit of hair on my tone. If I use any sort of drive, distortion, etc., even just that tiny bit of "pushed tube amp hair" sound, and am running direct (which is pretty much always these days), I find a good cab sim to be absolutely essential. It takes the harshness out of the tone and makes it warmer, more amp-like (clearly). Wouldn't leave home without it. And I like knowing the exact tone I'm hearing in my in-ears is what they're hearing at the FOH.