My thoughts,
The Marine combatives system (MCMAP) teaches restraints at a basic level, and I've not heard too many people talking about 'it's complete bs', so there must be some merit.
Something I truelly hate about the majority (not all, but the majority) of RBSD instructors/schools/systems teach only 'this is what you do when you're fighting some one who wants to kill you!' The Problem is, that kind of situation has a small chance of happening to anyone (like 5% of the US population will be violently attacked in there life time, that includes alot of situations when the smart thing to do is just give in; you're car isn't worth it). So, unless you're going out and getting involved with stupid people, and do stupid things to other stupid people are alot bigger then you, you'll probably never really need to know the exact amount of pressure needed to break someones neck.
That in mind, it does happen, and is something you should know. However, the odds are much greater that you'll be in a situation where someone is going to half attack you, without life threatening intent. Such as some drunk idiot swinging at you. In that situation, beating the person to a pulp is a bad idea. In that situation, the best thing to do is to restrain the person.
While joint locks and the lot do take more skill then just swinging, they can also be more effective. A single punch can hurt for only so long, but a single wrist lock can be applied and pain retained for as long as is needed. Also, I'm aware of no joint lock or restraining technique that cann't be turned into a break if needed.
That size can massivly inhibit the ability to lock someone is not true. Infact, the strength of the attacker can make a take down more effective/easier with practice. No attack can be mastered in a day, week, or month. It can take years of training to ever be truelly prepared for an attack, even if you are going with the simplest possible attacks.
Another reason to use joint locks is legal matters. The common idea of 'escalation of force' applies. There's no reason to pull out a glock and shot someone, when you can just as easily punch in the face, less legal problems. While 'not playing at there level' may get you more messed up, you don't run nearly as much of a risk of facing legal problems for excessive force. And generally, it comes off better to the cops if you tell them that you tried to restrain the person, before you cracked there ribs with a pipe.
Another though is weapons. Most of the restraining/joint locking techniques I know can be used to disarm someone if needed. Punching there face in cann't do that as well. And besides, if your opponent is armed, and you're not, you're screwed. As the saying goes 'don't bring a knife to a gun fight'. The same applies to a grab. The bodies natural reflex to a physical attacker is (generally) to tense up. So, if some one has a hold of your wrist (probably going into punch you), and you start punching them, they're probably going to hold on tighter, so if they fall, you might as well. In that situation, a wrist lock would be best. And there various techniques for when someone grabs you, and goes into punch you.
A final thought is against multiple attackers. When someone grabs your wrists from behind, and another attacks from the front, you're kind of screwed unless you have some idea of how to deal with that situation. Or if one person has hold of one wrist, and another person has ahold of the other. Someone else comes into attack, again, there are joint locks that can be adapted.
Do what you gotta do though.