The issue with playing multiple characters is you have greater difficulty committing yourself to all of them.
My brother, for example, doesn't have a set main and plays multiple characters, yet he puts the effort into all of them and builds on a foundation that is - for the most part - carried over from previous games. That said, he himself admits that his skill can very easily wane if he declines to use a character for too long; his Rosalina in particular was very rusty recently because he hadn't played her in a few months and it took him a few weeks to get back up to scratch with her.
Of course, I would advise against playing just one character, too. Especially with characters who have polarising matchups (Robin, Little Mac, Ganondorf, Luigi, etc), you're prone to dangerous matchups if you don't have a secondary to back you up. It's a double-edged sword: while you will be able to devote yourself absolutely to that character and thereby develop a deep, complex knowledge of how they work, you will run the risk of a counterpick from an opponent who exploits matchup advantages to compromise your competitive integrity.
The same rubric works here as it does with any technique or discipline: balance and practice. Work with what you've got and be the best damn whatever-it-is-you-wanna-be, but always make sure you have an ace in the hole for when you really need the help.