The stainless steel brewing vessels I own are for brewing and the lids do not seal.
I also want my mead to ferment in smaller lots so they are easier to move, adjust, test for pH, check gravity etc.
If I want to "tweak" my meads (add honey, fruit, spices, other stuff) I can do so more effectively in smaller lots which in my experience lends to better quality control and consistency.
It's much easier wrangle the 5, 7.5 or 8 gallon plastic buckets than it is to move a 20 gallon brewkettle. My brewkettles also sit on top of 250,000 btu natural gas burners. I wouldn't want to try and move 135 pounds of liquid around when it's time to go from primary to secondary, or to take them off the burners.
From the plastic fermenters I go to carboys now for secondary, and then I either keg or bottle. Mostly keg. it's easier than bottling, it goes into my refrigerator and stays cool, and it lasts a whole lot longer than a bottle does. ;D
Bottom line is the right tool for the right job man. The twenty gallon stainless steel brew vessels I own are not designed for fermenting, and it would be a waste to use them for that.
Derf is absolutely right when he said that you need a sanitary environment for your primary, especially when it comes to being able to seal your juice up.
Hope that helps,
Oskaar