More anecdata.
If I had a dollar for every time I've heard this one. I refuse to believe that professional landlords are keeping units off the market when the current rental market may well represent the top of what they can get for the next 5 years. Most of the long term landlords who bought 20-30 years ago have gone through at least 3 down markets from the late 80s to now. The current employment gold rush in SF is just that, and I suspect long term owners recognize that and want to strike while the iron is hot.
I am willing to change my opinion if anyone could provide any tangible data, but I only see this argument bandied about with no actual proof.
So in my rebuttal, I would say that I guess that there are less than 100 units like that across the city. I have no proof, but I feel thats the case, therefore rent control not bad. See how that works?