Worth fixing? Probably, for some people. Contact Evans and find out what a new frame like it costs and inquire about repair. I (being old skool) happen to really like high end steel bikes, a lot. A quick look at what high end steel frames cost new (let alone what coveted vintage ones sell for on fleabay now) will indicate I am not alone. While some might say steel lugged frames are a dime a dozen, can be had cheap, etc., (and this is sort of true) go price a high end steel frame (which this obviously is) and then decide if it is worth fixing. It will need a new tube and a paint job (but the paint job could be an inexpensive powder coat). Might be a good topic for the frame builder's forum as well.
Given no sentimental value it may not be worth fixing to you, but it sure looks like a frame worthy of saving. It is clearly a very nicely built frame.
Since you're in the SF Bay Area you might check with Sanner Cycles in Palo Alto
http://www.sannercycles.com/ since his frames are reasonably priced and he does frame building classes.
To be more specific:
1. Do not ride it!!!!!
2. Welding won't fixe it, it needs a new tube (easily done by a competent frame builder)
3. You could possibly sell it for cheap, but it is probably closer to a give-away since it will be a labor of love to have it fixed--if it will fit you well, I would say fix it and ride it, you might really be surprised!
If steel frame bikes are not your thing, then sell it cheap via the usual channels to someone who might want to fix it and build it up.