the actual biggest failing of Merry & Pippin's awkward introduction in the movie, imo, is the failure to explain that Merry&Pippin and Frodo are friends. in the book those are his 2 best friends; in the movie they literally don't directly interact prior to running into each other in the cornfield and it comes off like they are at best casual acquaintances.
this means that their decision to join up with the quest is a bit jarring. like. why not just go on home? they seem to go along with Frodo bcos that's what the book says happened and for no other reason.
however: i think this is actually a knock on consequence of another adaptional change, namely, the decision to make Frodo and Sam good friends.
in the book they are not friends but rather master and servant. they cannot be friends due to the class barrier that exists between them and over the course of the quest said class barrier is broken down; this is a reflection of the shift in British class dynamics that took place following WWI. some of their early interactions are hmm. well I think for a 21st century international audience they would come off a bit icky.
so: the time that might have been spent setting up the Frodo and Merry&Pippin friendship is instead spent showing Frodo and Sam being friends. in all honesty the fact that they are played more as friends whilst still being employer & employee actually makes the dynamic kind of Off in a different way (like. would you want your boss getting involved in your love life? i sure wouldn't) but as the movie plays down the master/servant aspect of their relationship in a big way i think most viewers are not troubled by this.
alas tho, I think the lack of explanation as to why in the world Merry & Pippin are there does trouble people lol!!
if anything i think the Bakshi movie handles this marginally better in that Merry and Pippin simply Appear as part of the group when Frodo and Sam set off on the quest. we can tacitly infer that Frodo invited them along and that therefore they must be his friends.
