Why was Boromir allowed to join the Fellowship?

[Inziladun]

Originally Posted by Juicy-Sweet

There's not much in those words that would make it so-so important to know the answer. No need to go to Rivendell to find out that "Doom was near at hand", they were all perfectly well aware of that.

The fact that both Faramir and Boromir had the same dream, coupled with the knowledge of the name of Elrond the higher-blood Gondorians would still have had, persuaded the rather insular Denethor to send one of his sons on a "secret" mission to have the dream decoded.

Originally Posted by Juicy-Sweet

So Boromir must have had a spritual side as well, I think some sort of vague notion/feeling that there was some sort of magical something at Rivendell, meaning not "weapons and soldiers" that could help them.

Not so much spiritual as a bossy big brother, unwilling to let his sibling take a long, hazardous journey like that.

[radagastly]

Considering his position as the "greatest" knight of Gondor and all, I suspect Boromir was also itching for a "noble quest" to test his skills and resolve, and was unwilling to begrudge the opportunity to his little brother. So he left Gondor to answer the riddle of this mystic dream.

As for why Boromir was included in the Fellowship, the reasons above sum it up nicely. Very likely, both Elrond and Gandalf had a feeling or sense that Boromir's participation would provide some kind of key turning point at some stage of the journey. Galadriel obviously did. How that would change the course of the quest, they would not have known, and perhaps only guessed. They would have seen it as important that it play out, at any rate, "For good or ill."