MacKenzy has a good point. A Rivendell is valuable enough for its design, ride, quality, and style that -- I think, depending on one's finances and responsibilities -- one can justify owning one simply for the pleasure of having a truly wonderful bike to ride; utilitarian considerations come second, or at least take a lesser place than, perhaps, with other bikes less distinctive.
As to riding it in harsh conditions: I've never ridden in heavily salted urban weather conditions -- I have ridden in snow enough to know something about what it is like; recall the snow packing up under the fender at the brake bridge on my first Road Custom on at least a couple of winter commutes to work. But this was rare and, in any event, no salt.
OTOH, I converted that custom and the next but one (2003 Curt) to regular commuting duties as fixed gears, in the first case personally removing unneeded braze ons, and in the second, having a builder do it more elegantly. (Conversion 1 actually turned out quite well, though.) In the case of the 2003, the clean, shiny bike was hanging on the wall while most of my cross-town commutes were on a (nice) beater, and I said, "Hey, that makes no sense," so I turned it into a commuter to enjoy it a lot more, and so it happened. This 2003 often got slung on cross-town bus bike racks, and locked up at public parking racks, so it got a few bumps and scuffs.
So, I think that, if I were in the OP's position, I'd have a Riv (or equivalent custom) dedicated to commuting, build it up to be as maintenance-free and cheap to maintain as possible (fixed gears wonderful for this), and simply be careful about washing and wiping it down in the harsh season.
OTOH again, I wouldn't trust to the oddity of a Rivendell to protect it from theft. And, depending on your riding preferences, you can certainly make a very wonderful commuter out of much less expensive and cherishable bikes. I've ridden many a nice errand beater, but the 2 I wish I'd kept are (1) the very early Raleigh Technium sports tourer, flexy, heavy, cushy, fixed conversion, great fun with (then) "fat" 32s (that's the one I used to pull the child trailer), and a pre-susp, NORBA-type top-end DB mountain bike, fixed conversion with White ENO hub and 60 mm Big Apple tires, 64" or 65" gear, and 46 cm Noodles. A really wonderful all rounder.
(Aside: I didn't own it during one of our more snowy years, but I did ride it in snow and on ice patches, and I can heartily recommend a British Racing Trike as the ultimate winter riding machine. Ice? No worries! It's also a 2-wheels-good / 3-wheels-better grocery vehicle, with a huge saddlebag -- mine was a Riv Hoss. And I wonder what a prospective thief would think at seeing a BRT ...?)
But none as nice as commuters as custom Riv road bikes!