From a historical perspective, I find Chabad's conclusion to be highly fallacious. Maimonides, after all, did a degree of such studying. He was a Jew in an Islamic caliphate. He even wrote on his two cents about Christianity and Christendom, which is hard to do if you are supposed to be forbidden from studying foreign theologies. It either means that Maimonides broke one of the very laws that he laid down for the general population, or *surprise surprise,* Chabad is once again incorrect. I'm willing to go with the latter on this one.
This goes beyond the Haredi community's authoritarian attempt to keep their congregants on their "narrow path" to make sure they don't leave the community. This mentality goes against the first commandment on the Decalogue, which is also the first of the Six Constant Mitzvot: Know there is a G-d (Exodus 20:2). As R. Noah Weinberg, z"tl points out:
We should not believe in God "on faith" alone. Investigate the evidence. Get knowledge. Research. Study. Analyze. It is a fundamental principle of Judaism: You have to know, not just believe.
Guess what this entails? Studying foreign theologies. Being in a world in which dissemination of information is unprecedented, we are faced with conflicting ideologies and religions. You can't just say "I have the truth" without listening to others. On the other hand, this doesn't mean we give equal credence to each ideology. For instance, I can eliminate Buddhism, Hinduism, Confucianism, Daoism, and Shintoism from the list because there is not a shred of positive evidence confirming anything they have to say. Even in the secular world, such crock pot theories as Communism can be dismissed because every time it has been implemented has been proven to be a total failure.
Part of knowing the truth is knowing that others don't know what the truth is. For me, it's not a matter of subjective "oh, I wish it were true, but I'm going to ignore everything in front of me." That level of irrationality is a denial of truth, and subsequently, a denial of G-d.
Some people in Chabad might realize that, as Jack Nicholson put it, people can't handle the truth, which can potentially be the reason why Chabad adds such stringency. Their approach ultimately becomes a double-edged sword. Yes, Jews might wander off to other religions because "it feels good," or they cannot stand whatever burdens an ultra-Orthodox lifestyle might thrust upon them, and abandoning Judaism is bad. However, by taking the mentality that studying other theologies is idolatry is, ironically enough, an idolatry in itself because knowing, not just whimsically believing on some flighty notion of faith, that G-d exists is the foundation of Judaism.
So yes, other theologies need to be studied so they can be disproved. The search for truth is what Judaism is all about. That is why it takes a certain level of intellect to pursue it. Since most people don't have it (trust me, Maimonides would agree with me regarding people's intelligence), it is best to find someone who has the intellectual capacity to help you walk through the complexities and nuances that reality throw out. May you objectively seek truth in all your endeavors!