It's certainly a fair question. In defense of the BofM, though, I'll ask the question another way. What doctrines are in the Bible that are not found in the other books of LDS scripture? Why do we need the Bible? I don't mean this to pit one against the other, but rather to say that the LDS view is that all these works combine into one set "stick of Judah, stick of Joseph". I haven't heard the term for a few years, but when I was young, we even used the slang term "sticks" for the complete set of scriptures (Bible and Triple :-). For example, "Hey, have you seen my sticks? I left them under the Sacrament Table after blessing the Sacrament in Junior Sunday School this morning."
Also, I think it depends on how any one individual views the scriptures as a whole. For me, I don't look for them to enumerate specific points of doctrine. I mean, they'd be a really inefficient way to do that. If that was their purpose, we'd start with the AofF, add about 50 more, and done. Prophet tells you to read the whole Book of Doctrine this year? Great, do it in one sitting - maybe during commercial breaks of football games. Rather, I look at the scriptures as the Gospel in abstract. Snippets, concepts, allegories, metaphors, similes. In them, I find my own truths. Because it's somewhat abstract, each person can hear in them what they need or want to hear... people can find what resonates with them. It's pretty frequent on this site alone, that I hear someone else's explanation of how they view the meaning of something in the Gospel, and I realize that people get different messages from the same source material. Not wrong or right, just viewed through a different lens. That's something I love about the scriptures: they adapt to the personal view of the reader.
For me, I like the NT and get the most from it. After that, sure, there's some good stuff in the OT, but that work is mostly a desert, with an occasional oasis. The D&C, the BofM, the PofGP... for me, they don't really do anything, but I also can find inspirational texts there if I try. But for others, it can be the opposite. A recent conversation at Church revealed that some people prefer the BofM and would rather ONLY read that and not have to study the NT as well. That boggles my mind, but my love of the NT over the BofM would seem just as weird to them.
From a historical viewpoint, I believe the BofM's key role in the foundation of the Church was to demonstrate that God is alive, that he's active, that he has a work in the latter days, that he is the God of the whole earth, that Israel is still the chosen people, but that those of that house are sprinkled all over the earth, that there is much that God has revealed before, that he is at it again, and that it will continue... that knowledge of God is at your fingertips and will continue to unfold if you seek it.
"Let us therefore no longer pass judgment on one another, but resolve instead never to put a stumbling block or hindrance in the way of another." --Romans 14:13