I'm not sure how much you know about tube flies, so I'll try to explain them a little bit, if I'm typing out a bunch of information you already know then I appologize.
You tie a fly onto a hollow plastic or metal tube instead of the shank of a hook. Traditionally tubes were used almost exclusively for salmon and sea run trouts, but more and more people are using them for steelhead, smallmouth bass, and trout. People are starting to tie nymphs on tubes, and a recent issue of Fly Tyer magazine even had some tube fly dry flies! So you have this fly tied on a hollow tube. When you go to fish the fly you first push a short piece of flexible plastic tubing called junction tubing over the back end of the tube. Once the tube is threaded on the leader and you've tied your hook on, you will push the eye of the hook into the opposite end of the juntion tubing the fly is on, that way you have a tube fly semi connected to the hook. The idea is that when you're fishing the fly the hook and tube stay together, and the tube doesn't just slide up and down the leader, but when a fish takes the fly and shakes its head the tube can come free of the hook and slide up the leader. This is an advantage because many long-shanked streamer hooks provide a sort of lever between the hook eye and bend for a fish to pull the hook out. So just to re-cap your system is a tube fly threaded onto your leader, a hook tied on the leader, and a piece of junction tubing pushed over the end of the tube and the eye of the fly to connect the two. This is also nice because you can use a short shanked, large gap very sharp hook (or even a circle hook), with a very long or large fly, and as you said, if the hook becomes broken or dull, no big deal! Just slide the tube up the leader, tie on a new hook, slide it back down, push the hook eye into the junction tubing and fish more!
Another advantage is being able to build a sort of convertible fly right on the stream. You could tie a mudler style spun deer hair head on a tube, and have an assortment of hooks with different colors of marabou winging tied on them, or rabbit strips. Get to the stream and decide what color mudler you want and build it! Another idea is building marabou leeches on the stream. Palmer a single color of marabou onto a short tube, be it purple, black, white, green, whatever. When you get to the stream combine as many as you want for a short, or incredible long, marabou leech pattern in any color combination.
I hope this gives you some idea of what tube flies are all about, but don't worry, if you want to get into it there are bottle tubes and FITS tubes and a whole world of new and confusing things! lol
This is a good site for some info and ideas about tube flies
http://tubeflyjournal.wordpress.com/2009/01/19/why-tube-flies/
