All of this stuff in the news about the polygamist sect in Texas has increased the general level of interest in Mormon religion and history. As a priest it is important to know something - even if it's very basic - about other religions, as well as other topics (such as philosophy). But with everything else a priest has to stay current with, and know, and do, it is simply impossible to become an expert in everything. There just isn't enough time in the day. One could spend a lifetime studying just one other religion!
It is important to know something about other religions, such as their history and how they began, and some of the basic differences between them and the catholic religion, but ultimately, as I learned in sales, it is better know your own product inside out, and be able to explain it, then to know some other product super well. You don't get "sales" when you bash someone else's product, or attack intricate details of their process (i.e. theology). "Sales" - in this case, convincing people of the truth of the catholic faith - come when you know, believe in, and can explain it well.
