But it's what the screenwriter intended the scene to do.
You're very wrong here, because plot is absolutely not the only thing that's necessary to a story. Stories are about characters, not just events. What's necessary is what serves the characters, their choices, their relationships, and their emotions. Plot is the result of characters' choices in pursuit of their goals.
The wedding was important to the film in many ways, even aside from the fact that it brought a resolution to the Riker-Troi arc that had been simmering since "Encounter at Farpoint" and brought to the fore again in Insurrection. The fact that Riker and Troi were leaving the nest and starting a family of their own resonated with Picard's longing for family, and that informed his interactions with Shinzon, whom he related to as a surrogate son of sorts and tried to offer guidance to. And the fact that Riker and Troi were moving on with their lives tied into the movie's themes about personal change and growth, about the importance of moving forward rather than stagnating.
But the dune buggy chase was not put there to serve character. If anything, it undermined character, because it's led to decades of fan complaints that Picard was out of character in his cavalier attitude toward the Prime Directive. Remember English class? Plot, setting, character, and theme are the four basic elements of storytelling. Some sources also include conflict, though I'd call that part of the plot. We've already eliminated character. The chase scene doesn't serve the plot, because it's totally forgotten once they're back on board. It doesn't serve theme, because it's just chasing and shooting. It doesn't serve setting, because its desert setting exists only as an excuse for a dune-buggy chase and is never seen again after that one sequence. And it doesn't serve the film's overall conflict, because the superficial conflict that does exist in the scene (the shooting and chasing) has no relevance to anything else in the movie.
So there is literally no story reason for the scene to be there. It was tacked on as a lazy excuse for an action scene. Now, I can live with the attitude that big-budget sci-fi movies need action, because that's what audiences expect. But there must have been a way to craft an action scene that served some greater purpose than just meeting some arbitrary quota for action scenes.
For instance, do the whole sequence differently -- have Data learn that he has a "brother" somewhere and have him put himself at great personal risk to liberate B-4 from his captors. That way, the action serves character because it shows how much it matters to Data to have a brother; it's an actual obstacle to his goal of finding his brother, rather than an afterthought tacked on after he's already found him. And it serves the plot because Data's determination to save B-4 underlines the difficulty of his later decision to shut down B-4. Also, maybe the captors could be Romulans, and after B-4 has been liberated and those Romulans are chasing him, the Scimitar shows up and fights them off, and then the Viceroy informs Picard and Data that those Romulans worked for the old regime that Shinzon's now overthrown -- and that leads to the invitation to Romulus and gives Shinzon some bona fides.
That way, you get the obligatory action sequence in Act I, but in a way that's organic and purposeful to the story, a way that serves character and advances the plot. Instead, they took the laziest possible path and stuck in an action scene that merely interrupts the story and has no connection to anything else in it. It was badly done. There's no way around that, because it would've been so easy to do it better. I mean, it took me less than a minute to think up a better way to do it, a way that would contribute meaningfully to the plot and characterization. So why couldn't they?
(Okay, they couldn't because Patrick Stewart wanted an excuse to drive around in a dune buggy and he had enough clout to make it happen. But that just underlines the gratuitousness of it.)
