What was the point of Oldtown?

[Lockjaw of House Boltagon]

Was it just so Sam could cure Jorah and Gilly could find that lost copy of the Winds of Winter? 

Because that seems...like a waste of a perfectly good Jim Broadbent. Also, Sam was totally like that spoiled kid who lasts a semester in college and goes, "That major wasn't for me." I mean, I get that those dusty old farts were frustrating but they did say they were going to ask the Winterfell maester for more proofs. Sam could've stayed a bit longer and tried to convince them instead of leaving in a huff like a diva.

So, is this the end of that plotline or do you think the Citadel still has a role to play? 

[Free Greatjon Umber!]

Yeah it seems like they didn't know what to do with Sam once he got there. I guess it was so he can get some books and scrolls that may be helpful later? And yes, to save Jorah I guess. Since Sam is officially gone I doubt we will return to Citadel. The only reason I think they will ever become important again is if they verify the information and try to convince the southerners to send men north against the army of the dead but since Jon and Co. are getting a wight the Citadel seems unnecessary for that function, so I think its unlikely that they will be of importance.

[Mikkel]

He needed to pick up the Dusty Tome of Plot-Reveal+5 and the Random Scroll of Just-in-Time Pertinent Information.

I don't think the basic concept of Sam getting fed up and leaving the Citadel is necessarily wrong, it's just that everything is so incredibly rushed (just like a lot of other plotlines this season), nothing has time to mature or even settle before its on to the next thing, so it ends up feeling pointless.