Spirit does not have unlimited jumpseat. Use to be that way several years ago. it has since changed, as it should. We now offer cabin jumpseat priveldged to F/As(and even pilots) from other airlines that we have a 'reciprocal jumpseat agreement' with. This does involve several carriers. Off the top of my head I can name..jetBlue, Frontier, AirTran, Skywest, Gulfstream, Carribean Sun, USA 3000, MaxJet, ATA. there are more but these are just off the top of my head.
Some airlines (like Frontier) will allow our F/As to actually occupy the extra cabin jumpseat if no seat are available, as we do with all "flight crew members" If there is a pilot trying to jumpseat and either both(or MD80=1 jumpseat) jumpseats are occupied and the flight is full we will let you sit on the extra cabin jumpseat. ive had many a pilots share the jumpseat with me as a working crew member. never a problem with me. I know what its like to commute, so yall are always welcome to share my jumpseat with me!