A good friend of mine is a special forces combatives trainer. I asked him how much of their training is devoted to full-speed and full-contact simulated sparring. He told me "no more than 5%". For him it was a matter of balancing value and risk.
Full speed, full contact sparring has the value of exposing trainees to some of the aspects of the freedom and potential for chaos of real world environments. It can be used to simulate aspects of intensity and to train certain types of strategic and tactical skills.
One of the main limits of simulated combat is that it is still that: a simulation. It's a type of simulation that has a hard time approximating the real world, even when you truly endeavour to do so, and as training environments go it is not focused enough for efficient skill development. You just don't get enough reps of any given skill (or psychological mode) to make effective forward progress, when compared to other training options.
When this is compared to the risks of these environments, especially when working with soldiers that need to be deployed at a moment's notice, my friend (and other trainers like him) find that the risk of these environments (even just in body strain) are not worth the value that you can get from them.
So Why Spar?
Because it's fun. Because it's what you enjoy. Because it's a piece of the pie that you want to eat. Because you want to see what happens when you're there and use that to inform your general training. Because it's a way you like to connect with others. It's a thrill. It gives you a reason to travel.
These are all awesome reasons.
The main thing I want to dismiss is the argument that it is a training mode that is somehow more "real" or more "effective" than other types of training or other expressions of the art. There are brilliant martial artists who only do forms, who only spar in particular manners, and who only drill. These all have value. Even within the pursuit of being as effective a martial artist as you can be with the "real world" as your chosen battleground, know that you're in good company (Special Forces!) when pursuing a program that de-emphasizes full gear/full contact work. You'll certainly be welcome on my training floor.
Hone your craft in the ways that bring you joy and take you down the path of mastery in the way that gives you the most fulfillment. Happy training everyone!