Your post only makes sense if God does not exist and if His prophets are not real. If the opposite assumptions are held, as they should by anyone of most religions and most especially someone that is LDS, then much of what you said falls apart. As if God is real and a book actually does contain His word then contradicting that book would be going against what is real and therefore not rational.
So it is not because an old book said so but because God said so and since it is God that said so one is able to confirm for oneself the truthfulness of not only the “old book” but also of the claim that it is wrong to forbid others from eating meat (or whatever else). Since God is all-knowing and we are not then when our limited understanding contradicts what God has said should we claim that God does not exist despite evidence that He does or should we think that we are mistaken in our views?
If God exists then that which is predicated on His non-existence becomes irrational and the rational thing to do is to change those views that contradict His. If there is no God to listen to then it is clearly irrational to listen to God, as He wouldn’t exist in that case. However, if there is Something to listen to then why is it irrational to listen to It?
Many of the conclusions drawn on this site depend on the non-existence of God and fall apart if He does in fact exist (and one is able to verify for oneself that He does). However, even with the existence of God it is still extremely useful to learn to be more rational (the stated goal of the site) as God does not command in all things but expects us to govern ourselves based on the principles He has given. I wish all men everywhere would take ideas seriously and attempt to be rational in their beliefs and in their actions.
More converts are obtained then are born into the church. Since missionaries are in pairs then last year there were an average of 10 converts per missionary pair. Does this count as many or few?