Spirit Airlines tends to operate a thin and infrequent network of flight. By "thin" I mean they only have one option to get you from point A to point B. For example, if you want to go from Richmond, VA to Orlando, there is only one non-stop flight in each direction. There are no other options to connect in another city.
The "infrequent" part means there typically is only one flight between city pairs each day. And for some city pairs, Spirit only operates 1 or 2 flights per week.
If everything goes according to plan, the only thing you have to worry about is cramped seating. When things go awry, your entire trip can get ruined. According to their Contract of Carriage, if Spirit cancels your flight, their only obligation is to 1) refund your money, or 2) put you on the next available flight. If you try for a refund, you'll be stuck buying a last-minute ticket on another airlines. Doing that will certainly cost a lot of money.
Option 2 isn't much better because Spirit's thin and infrequent network means you have almost no options. They will only put you on the next "available" flight. That may not be for several days.
Now, let's contrast the options available with Southwest in our example city of Richmond. If you happen to be on the first flight out and it gets canceled, Southwest published 5 other itineraries that will get you to Orlando on that same day. All those options means your trip probably won't be a bust just because your flight is canceled.
Personally, I won't fly Spirit...even though they are based here in Fort Lauderdale. My one attempt resulted in a travel headache. I view them a bit like gambling. They are fine until you have a problem. Typically, fixing that problem will cost you a lot of money (unless you completely abandon your trip). It only takes one problem to negate any savings you may have created.
(Hopefully, I explained that clearly.)