Recency does play a part but it only helps a character get noticed, beyond that it's relatively meaningless (I say relatively because we don't know about how deeply Sakurai thinks about character additions).
Big example of this would be Sakurai's recent admission of how he selects Pokemon characters by looking through recent Pokemon media. However, it seems that next to no one fully read through the entire article to see what he said in addition:
find out which ones are going to be central to any of conversations in Pokemon going forward."
"But it’s not just that – going back to just what we talked about, what’s unique about them? Where do they fit in with the rest of everything else? What do they have? It’s a combination of those things."
If this doesn't confirm that the "Flavors of the Month" theory is ridiculous I don't know what does.
He's looking for memorable and unique characters, not ones that he can just easily replace with each and every game. Roy & Pichu getting into Melee, and Lucario & Ike getting into Brawl while Roy & Mewtwo were cut is just a simple coincidence. If you know anything about Pokemon & Fire Emblem in recent years, you'll know that Lucario & Ike are actually a LOT harder to cut than one might think (Lucario, who's had several Gen 5 appearances, and Ike who's received heavy nods in FE13 in addition to his descendant Priam).