I've always liked the idea of a backup pump, but you can't really be confident that you can (a) suck routinely through the spare, or (b) push through the busted mechanical. With an in-tank pump, I'm not sure if you could plan on sucking fuel through the busted one either.
Changing a mechanical pump on the trail would not be fun. I know people do much worse jobs, but I've always found installing those kind of frustrating. Keeping a spare electric pump on hand and then plumbing it in-line doesn't sound so bad to me though. For TBI you'd be good to go with that. With a carb, you'd probably want to run a new length of hose right to the carb and bypass the mechanical pump.
FWIW, the Carter pumps will self prime like champs, but the Ford boost pumps that some people use will not suck air, at least in my experience.