The multiple attacker scenario is not a good one to be sure, but to not train for it is irresponsible if real world self-defense is your goal.
Consider the knife, you are absolutely at a disadvantage, so do you NOT train for it? Dan Inosanto once said, "All the knife training I give you may only give you one chance out of ten to survive, but it's better than none." The same can be said for multiple attackers. Don't train for it or expect it and you may have no chance at all.
Avoidance and intelligence is your best defence, of course, but sometimes fate will have it's way with you. If you do have to face multiple attackers you may end up wishing that you had prepared for it to begin with.
It may be your personal choice to carry a weapon or self-defence device such as a chemical spray. Knives, guns, batons, stun devices, etc. are all good options, but you may also escalate a situation to a far worse one than it was originally. You have to make that judgement call for yourself.
If you do have to fight multiple attackers and do it unarmed, you have to fight like your life depends on it. It vary well may. A group of two or more can easily kill you and not even intend to. The mob mentality can make it an even bigger risk, especially when you fight back. Don't take it that seriously and you may pay dearly for it. Your choice.
Rickson once wrote an editorial for either BB mag or IKF, I don't remember. What I do remember is that he stated that he would never want to go to the ground in a multiple attacker situation. He said that he would keep moving, throwing, punches, kicks, elbows, etc. and keep them off of him.
As was stated earlier by others, your attackers intent is definitely a factor. If you are well trained you will usually have a decided advantage over the average person. Once they see that you can fight, many will back off and decide that they don't want to fight that bad. Many are "tough guys" as long as they think they can intimidate or win easily. Once they realize that you can hurt them, their goal will soon change.
I've been in a few multiple attacker situations. Most of these were two or more guys that were confrontational until they realized that I was willing to fight. A few situations were fights-to-be until you hit the first guy. I've been in one 4 on 20-25 situation. Not cool. Luckily, they either didn't want to fight that bad, couldn't or were just to drunk. Either way, we were lucky enough to walk away from it.
If you are able to, you want to keeping moving, sectoring and striking so that you only face one man at a time and don't allow the others to flank you. If you are surrounded, you now have the "advantage" of striking in all directions. You always seek to break out of the circle if possible and work your flanking again. In the circle, using hair and clothing can also help. Jerking an opponent to the ground or into his buddies momentarily gives you openings or at least stops them from trying to hit you on that side. Pushing, shoving, pulling, jerking all while striking and moving and trying to bust out of the circle is how it goes on the inside.
We work drills in the gym for these scenerios to give the students some experience at this. NOTHING kicks your ass worse. Nothing. If these drills serve no other purpose, they teach the student how much they DO NOT want to fight multiple attackers.