Wow, I can see from the comments that most people seem to only do
day sailing. Which is great and if that is all you do and can always pick your
weather, then I wouldn't have a removeable stay sail as well, because you wouldn't use it often enough to warrant it.
On the other hand, if you do passages, the
weather inevitably will be rough at times. I have approximately 40,000nm on
passage experience and have a 45' Halberg Rassy with a removeable inner forestay that sits alongside the mast when not in use. I have sailed in winds up to 55kn for multiple days on end as well as sailed to windward in 30-40 knots fairly often. When going to windward if you have
roller furling, after
furling your
jib to a certain point what remains will be too flat to be effective and will be only be adding to lee
helm. You soon get to a point where you cannot get rid of lee
helm (a
boat sailing with lee helm is looking for a knock down!) unless you fully furl the headsail. Then sailing under main alone has its own issues.
I found in heavy weather, having an inner stay that I could put in place being invaluable. To windward around 25 knots, I would stow my
jib and connect the inner stay to put a staysail on. The
boat balance would be remarkably improved as well as sailing comfort. Reaching or downwind somewhere between 30-35knts I would do the same. In 55knts I could sail quite comfortably under storm staysail and trysail on a wide to close reach.
If you find yourself in weather bad enough to warrant heaving to, you will find it a lot easier and more comfortable to heave to under this same configuration.
Having the ability to store the staysail alongside the mast when not in use has the obvious advantage of ease while tacking.