Some thoughts on this question for others thinking along the same lines. First, no lens can be all things to all people so what are you primarily photographing and what will you ultimately do with your photos? Primes are generally more clear and 28mm on a crop camera would take you nearer to the much prized 50mm focal length. Zooms also have more compromises and the 18-55 IS kit lens is also slower (f3.5 to 5.6 if I remember correctly). Overall this zoom has fairly good optical quality. Also do these primes have image stabilization like your kit lens? Not an issue for me as I shoot on a tripod most of the time and often use an off camera shutter release, but do you? Are you used to walking around without a tripod and just taking shots? In this situation the IS may provide that little bit more you need to get a clear shot.
What will you be photographing most? The 18mm focal length means you'll be able to get wider angle shots than with 28mm. What if you want to shoot a landscape or a large group of people, will 28mm be adequate? Yes, you have to zoom with your feet which isn't always possible depending on the situation. I found myself pinned in by a large crowd and couldn't move to get to another intended location to get my shots with my prime. What if you're in a location where you can only move a short distance such as some overlooks or scenic view spots? Will a 28/35mm be adequate?
Regarding low light conditions. Definitely having a faster prime lens will produce clearer, nicer shots. But, for example, if I'm photographing a city night scene and catching some light streaks from the passing cars using a long exposure I'm usually set around f11 - 14 so I don't need to go down to f2.8 to get a nice shot. Fairly inexpensive off camera flash/lighting can be used to light your subject or freeze motion. I guess it depends on your style of shooting, what's acceptable to you, what your budget is, if you will be making enlargements and what you want to photograph. Will you be showing these mostly as small images to be shared via social media or websites? Then perhaps you don't need as much clarity. And these days with phone and iPad cameras and such people don't seem to mind blur, grain, etc. You may even want some blur such as when getting that dreamy look to water in low light. Or more grain if that suits your style.
Remember too software is more sophisticated and some corrections can be made even with relatively low end software for grain, etc. Though it's not as good as getting it right in camera if you are on a budget this can be useful. If you're concerned about your wallet you can get older versions of good software at reasonable costs if you search/wait for the right price. I purchased Lightroom 3 not in box, unused and unregistered for around $35 some time ago and it works just fine for my purposes. They are coming out with Lightroom 5 so the price on older versions will probably drop if you just wait. I think there is software you can use at some of the photo websites like Flickr. Just consider your options and I totally agree with not getting rid of this particular kit lens. It may come in handy to use along with your prime and if you're used to shooting with it you're that much ahead.