You are correct in your assumption. If you buy the calibration mic you get a trial of the reference software to calibrate your room which lasts about 15 days from memory. Once the demo period is over you are left with the profiles you saved. These can be used in the vst or in systemwide. If you subsequently change the room and the profile is no longer accurate, you would need to buy the full licence for the reference software or wait for a deal.
The headphone reference software is slightly different to the speaker software in that you have a load of calibration files that come with the software which could subsequently be used in systemwide if you have one of the models that has a profile. Of course if you don't have either a calibrated mic or headphones with a profile, systemwide still works in that you can use the simulations to change the sound (from memory).
The beauty of systemwide for those who have useful calibration profiles is that you can apply correction to streaming sources so that they can take advantage of your calibrated room or headphones. Before that you could only apply profiles to daw output via the reference vst plugin.