I used to play as solely Marth for a long time. He's my main because I enjoy his playstyle and he's the character I have the most confidence with. Still, there's a reason I started adopting new mains. I started playing as Captain Falcon after I watched the Smash Brothers Documentary and saw what a legend Isai used to be with the character. Then after seeing how amazing other players are with the character like Mango, Darkrain, Silent Spectre and Scar, I was convinced that Falcon could be the best character if I could hone that combo ability. I haven't got there yet of course, but it's become my new goal as a Smasher.
I later started playing Sheik just because of how good her edge guarding is and how other Marth players (like M2K & KDJ) also use the character. I'm probably the least experienced with this character, but if I keep practicing, I'm sure she will become a powerful asset.
Before I even knew about the competitive side of Melee, I used to play as Mario a lot. I haven't played as him in a while now, but I still enjoy his playstyle, so I adopted to play as Doc Mario. His slower but more powerful playstyle fits me a lot better than Mario's which requires a lot more work to get good at. Doc isn't really someone I'd use for tournaments though, unless I someday get ridiculously good and I'm feeling cocky. For now, I just play as him in friendly matches.
Personally I think having a small selection of characters to main is a good thing since now, you can change the matchup whenever you feel appropriate. By only having one main, it will be very easy for players to learn your playstyle and use it against you.