When I look at the 15 bbl tanks in brewpubs or other small breweries, it would almost seem more natural to ferment in a SS keg. I can understand the geometries are different, and perhaps those brewers have a few moves we don't, but in the end they are guaging the fermentation process based of the CO2 release (bubbles) and the Specific Gravity.
Obviously our goals are to get out of cleaning and reduce the possibility of contaminants, make good beer and save time wherever possible. Generally I like to produce beers with primary and secondary fermentation phases, and in some cases I filter between the secondary and the serving vessel. The filtration operation is done under pressure and requires me to move from Secondary into a keg if the secondary fermentation is not done in a keg.
Doing a primary in a keg, begs the question - would the sludge in the bottom of the tank be effectively separated from the beer or would it move with it? The fermenters in the breweries seem to have a conical bottom, and the "beer out" opening is just above the sludge level. It crossed my mind that perhaps we'd need to take the same mentality, and tip the corny to the side during fermentation (at about a 30 degree angle) and that would keep any sediment from being under the draw tube. This would be done at the expense of leaving some beer in the keg perhaps.
Next hurdle is a chemical question. If the beer is moved from Primary to secondary using CO2, will there be enough oxygen in the secondary for the fermentation to continue. In a recent batch of wheat beer, I moved from primary in a carboy to a corny keg for the secondary. The beer was moved by siphon, not by pressure. The keg, however, had been charged with CO2 right up to the moment I began to siphon into it. For whatever reason, I could not get the fermentation to resume. The beer was still good but cam out with a higher than target specific gravity (lower than target alchohol content) Could be a fluke, but it did open the question for me if the secondary was short on oxygen, due to having head space filled with CO2.