Yes, if you bring your expectations WAY low, and know what you're getting into, I suppose that's fine. But people really seem to expect that they're getting just a "no frills" airline for a much cheaper price, and it's far beyond that.
The "seat pitch" on Spirit is only 28 inches -- which is uncomfortable for adults who are 5'6", and impossible to keep your legs straight for someone of my height (or anywhere near my height).
Frontier's pitch is 28-31, depending upon the seat.
Southwest has a 31-32 inch pitch, which is standard. However, "standard" has been shrinking over the years, even though US men are rapidly getting taller on average.