Ahey,
I believe this is generally considered a good idea, yes.
On a more serious note, yes, most tanks I know of find themselves needing off-spec gear of some kind, whether for healing or for DPS. While there is definitely a shortage of tanks, they also happen to be the least represented role in a typical raid group, which means that raid viability is
very dependant on having the
exact number of needed tanks available.
It stands to reason that, the more tanks we have that can off-spec as something else, the more flexible we are, and thus, the stronger our team is.
Cheers,
J.