I've flown Spirit to Orlando several times in the past and never had a flight get changed. My guess, and this is based on my experiences, is that the ACY to MCO flights are very popular and usually have very full airplanes, thus no need to change or cancel flights.
That being said, my parents have used Spirit to fly into Tampa and they have had flights changed and/or cancelled.
As for whether it's worth it, that depends on the price and if you're comfortable with it. This far out, I'm sure the prices will fluctuate many times before August comes around.