Cartridge hubs vs loose-ball hubs My LBS recommended cartridge hubs to me. Here are my doubts.

Seeing as how I never ride in the rain or any such wet substances, I don't really need the "completely sealed" feature.

Sheldon Brown says that if taken care of, the body of a loose-ball hub will last indefinitely. (not the balls and cones). So if you maintain your hub, you should be able to indefinitely avoid having to re-spoke your wheel.

Sealed bottom brackets make sense, because it's easy to pull your cranks off and swap it out. But if you have a cartridge hub, when it wears out you will need to re-spoke your wheel. Am I right about that, or do cartridge hubs have a system for just replacing the bearings?

Also, is it possible that a cartridge hub would last as long as the rim? Then you would just replace the whole wheel once it started having problems.
Most hubs these days claim to be "sealed".  This basically means that if you look the hub from the axle end you can't see the balls of the ball bearings, because there is, at least, a "dust shield" that leaves only a very narrow opening between rotating hub and fixed axle.

There are also "cartridge" hubs, which were the original sealed hubs popularized by Phil Woods, back when standard bearings had no dust shields and were pretty crappy.  These use a (theoretically) replaceable cartridge on each side of the hub, where the cartridge is an industrial bearing cartridge similar to what's used in electric motors (and similar to what's used in bottom-bracket cartridges).  However, replacing the cartridges requires special tools, and likely many bike shops would just give you a dumb look if you talked to them about replacing the cartridges.

Both of these schemes are a significant improvement over the original open bearing style, since, not only do they keep most moisture out, they also keep out dust and dirt.  For the average rider, though, the cartridge scheme is probably not worthwhile, since it is not possible to simply rebuild the bearings.