I know that Iain has covered this in great depth in his podcasts, but I want to add my two pennyworths to the argument as to why the floor really isn't your friend.
Just before I explain how I have come to this decision, let me tell you a bit about me. Yes, I am a martial artist having studied Karate for well over 20 years, I have also dabbled with Aikido and Judo in my time. However, on top of this, I am an ex-prison officer dealing with some of the most violent, aggressive thugs (juveniles) in this country. During that time I was trained as a use of force instructor so taught others the control and restraint stuff. I am now a serving police officer. So you could say, I have had my fair share of violent confrontation and know the laws around this area pretty well.
Now, I don't want to be seen to be having a go at BJJ or other grappling arts as I am not. However, any system that purely trains for the floor fight is flawed massively, you are essentially reliant on 2 things from the great members of the public:
1. You go to the ground and manage to get the assailant into a hold whereby they can't escape; great, now you just need someone to kindly call the police to come and sort it out
2. You go to the ground, there are loads of people about, you better hope none of these people are your assailants mates as they won't be best pleased.
On top of this, if you are going to take a fight to the ground, you have got to be concerning yourself with the welfare of that person as well (I know this sounds bizarre, right). Seriously, if you take someone to the ground, pin them there and won't let them get up/they can't get up, then you are in a world of problems if that person starts to go limp....positional asphyxia (placing a person into a position that they cannot easily escape from that interferes with the mechanics of their diaphragm/breathing) is a huge risk in these situations.
In control and restrain situations, you take a person to the floor to control that person better, you have (in a custodial setting) a minimum of 3 officers looking after different limbs, ALL looking after the welfare of that person. In a "fight" or self-protection environment; you won't have this luxury, generally you are on your own. If the person you are trying to restrain on the floor starts to have breathing difficulties, what do you do? You don't let him up right? In case the person is faking it and goes again, so you hold them there and they pass out....OUCH world of pain legally right there.
I always, always kept people upright if I could, if I couldn't then I got them upright as soon as possible to minimise risks of death to them (I don't want to be the one in front of a jury/inquest thank you very you much). When I trained prison officers, I always try to get them to deal with things upright and not take something to the floor unless the appropriate staff are present. In a self-protection scenario on "the street", you don't have this luxury.
If by chance you end up on the floor then I wholeheartedly recommend getting up again as soon as possible. Criminals don't care if you end up worse off than they initially thought, if they can get what they want from you and make good their escape then they will.
Criminals don't fight fair, they don't start if they don't expect to win and in general, criminals tend to hang out with other like-minded criminals.
Sorry, just wanted to put my bits and pieces across
Oh, and some really well built/muscular people can't punch for toffee...the amount of broken hands I saw in the prisons because they had all the muscle but just didn't know how to form a fist correctly was insane!