Despite the team eventually splitting, the Fellowship was tasked with the most important goal of all in The Lord of the Rings: destroy the One Ring. It was crucial that every member served a purpose, as they were likely to face constant enemies and obstacles, all while remaining hidden from Sauron. And so, why was it that Merry and Pippin got to join?
Their initial team-up with Frodo was pure coincidence, as they just happened to bump into each other in Farmer Maggot's crops. And with the Ringwraiths in hot pursuit, Merry and Pippin thought it wise to stick with Frodo and help him get to Rivendell. While they were able to successfully reach Bree, the two mischievous Hobbits did mess things up the rest of the way. For example, their cooking on the side of Weathertop is what alerted the Nazgul to Frodo's location.
While they were undeniably kind and charming Hobbits, Merry and Pippin didn't have much to offer, at least on the surface. They were often clumsy, possessed no fighting experience, and most of the time seemed to have no clue what was going on. Yet, they made it into the Fellowship, which means Gandalf saw something special in them.
Within the Peter Jackson movies, Merry and Pippin casually joined the team, and everyone just accepted it. However, in the original novel, the choice of who joined the Fellowship was far more debated. Gandalf and Elrond had settled on there being nine members, as it was large enough to defend Frodo, but small enough to remain undetected. And the first seven were chosen with relative ease, as it comprised fighters and royalty from across Middle-earth. But they were unsure who should take the last two places.
This is where Merry and Pippin hopped in, yelling, "We don't want to be left behind, we want to go with Frodo!" Yet Elrond immediatly rejected this idea, saying that they don't know the dangers, and that Elves from his home would be better suited. But surprisingly, Gandalf stepped in to defend the Hobbits, saying, "I think, Elrond, that in this matter it would be well to trust rather to their friendship than to great wisdom." Meaning that Gandalf thought it best for Frodo to bring friends along, rather than all warriors.
The wizard continues, explaining, "Even if you choose for us an elf-lord, such as Glorfindel, he could not storm the Dark Tower, nor open the road to the Fire by the power that is in him." But still, Elrond disagrees with Gandalf, believing that talented fighters with greater wisdom should be sent. And it's hard to argue with his concern, as any extra sword would've greatly eased their battles against the Orcs.
However, Gandalf also makes a convincing point, as he believes that no matter how skilled the final two members are, it won't change much when going against the armies of Sauron. And so, he thought it was more important that loyal, kind-hearted friends join the group -- people who Frodo could undoubtedly trust. And regardless, it's not as if Hobbits are entirely useless, as they could easily remain undetected, and were far less susceptible to the One Ring's corruption.
In the end, the decision to bring Merry and Pippin along proved for the best, as without them, the Ents never would have stormed Isengard. Yet it's still easy to understand Elrond's concern, as with the importance of the task, he believed it was more important to have people defend Frodo. Gandalf, however, being as wise as ever, saw the importance of loyalty and friendship above all else... even if the two Hobbits did annoy him most of the time.
Peter Jackson's The Lord of the Rings trilogy is available to stream on HBO Max.