This is indeed a concern because it can be used for many other drugs. However, it's very difficult at this point to gauge the importance of this behaviour. One has to consider that they can do that not only with electronic cigarettes but also with many other things. The issue of the consumption of other, in this case illegal, drugs through some existing legal apparatus like an electronic cigarette requires a broader approach to enforcement.
It's not only a problem that pertains to electronic cigarettes. It may be compounded, however, by the fact that these products are now seen as very normal, at least in certain countries, and therefore pass totally inadvertently for others, in terms of enforcement in regard to the illegal use of drugs.
My point is that this is not a problem only with electronic cigarettes.