The problem with a secondary is that it forces you to develop multiple reflexes for even basic things. For instance, different wavedash timings, different ledge dash timings, different dashdance lengths, different tech chase setups... in short, picking up a second character makes it much more difficult to have clean movement and tech skill unless you've already mastered Marth.
As far as how much Marth needs a secondary... it's not strictly necessary, but it can be helpful. Marth can beat every character in this game, but all of the top tier matchups can be tricky to some extent. He's also noticeably weaker on Dreamland in Bo5 sets, especially against floaties. I would recommend playing Marth alone for a while, figuring out which matchups you personally struggle with, and picking your secondary based off of that. For instance, I dislike playing against other Marth's and Falcon's, so Sheik would be a reasonable secondary for me. If you found yourself struggling against Sheik and Peach, you might pick up Puff instead.