The cities I listed as having lived in were actually three (I realize that I was unclear about California, where I worked in SF but actually lived in San Jose). I can break it down to under a year in one place and at least six years in each of the other two places. Even there, I actually could have probably found a way to not move, so it wouldn't even have been an issue. (I've had the same employer for almost fifteen years).
So, in my case, it was only a perceived need for mobility and unwillingness to say "I'm here, for a long time" that kept me from owning a home and it was probably not the best financial decision, looking back. I sacrificed a lot of income over the years for something I thought I needed, but never did.
It is definitely a bigger concern, now, than ever before. I absolutely concede that point. My grandfather built his home and lived there for more than fifty years. I will be happy if I keep my home for ten or fifteen. Our lives and careers change too often, now. Buying a home now requires the added consideration of whether you'll still be here in five or ten years and how many times (especially in technology) has taking a new job also required moving? (By the way, I think they say it tends to require about five years of ownership to compensate for the costs of buying and selling a house - and that was back when houses were increasing in prices, instead of decreasing).
Of course, if home prices reached a naturally lower point instead of being artificially kept up, then the affordability may trump all the other concerns. Hell, if I could own a nice home for $100k, I'd buy one in all three of my favorite cities and never have to worry about uncertainties of live and mobility again!