I used to have an SL4 and agree it can fit nicely in a pocket, and was a nice little light in it's time. The UK1200 was a perfectly fine light, but it's rather bulky to travel with, not to mention the weight of all those D batteries.
Personally I wouldn't buy any halogen light now, as I think LED is so much superior at this point in brightness, color, and battery life. You should be able to find used halogens cheap as I think many people have upgraded, and for some people it might make sense to pick up a deal on them. But if you dive enough, your savings will eventually be eaten up by all the batteries you have to feed them, unless you can use rechargeable batteries. So I think they're only worth buying if you don't use them much, or they're dirt cheap and you don't plan on them as a long term solution.
As far as the SL4 as a handy backup, I agree except for one thing - in the Caribbean, you may find that you also like to use a small light during the day to look in holes and under ledges. If you do much of that, you now have a backup with batteries that are rundown, unless you bring enough batteries and swap them out. And most small bright LED lights will be better at the "peeking in holes in the daytime" task.
