Re: aikido vs jiu-jitsu
I would not worry about running into someone like Royce Gracie in a street fight but, like Matt said above, you could easily end up against a former high school or college wrestler. Having some ground fighting skills in my view is very important. Even somone with no wresling experience who is bigger and stronger will likely squash you on the ground if you are not prepared. And please leave suwariwaza out of this because it is pretty silly to think this would work against even a beginner in any grappling art. Suwariwaza as far I was told was a way to refine your standing technique by emphasizing hip movement, not a realistic form of ground fighting.
Those that say "well just dont let yourself be mounted" or "don't go to the ground" may be issuing good advice but it is only an ideal and not always realistic. If someone with grappling experience gets their hands on you in an attack you have a very high chance of the fight going to the ground. Then what? I guess you are in big trouble.
In a multi attacker scenerio it would surely be pretty stupid to deliberately bring the fight to the ground. But having the skills to escape from pins, get in a superior position, break people's arms, release grips, choke people out etc. is incredibly valuable because these techniques give you a chance to break free and stand up to meet the other attackers. If you did not know any of this you are pretty much going to be pinned and probably kicked until near death.