I don't think the necessarily have to have a very useful use when combined together. It has a use by itself but that doesn't mean that combining with Duraluminium is going to mean it it's particularly useful.
Bendalloy has it's own allomantic effect that is independent of how well it works with Duraluminium and same with all the other metals even if they do have amazing effects when combined with Duraluminium.
EDIT: I don't think you would even get super old the metal would be used up in an instant like with all other uses of Duraluminium and after that the bubble would collapse, you wouldn't be in the bubble long enough for you to age. And that instant that you spent in the bubble would be a minuscule amount of time for everybody else, so while I don't think you would age it still doesn't have that much of a use.