Ditto to what others have said. The only major downfall to having multiple classes is grinding for class materials (eg. Plasteel plating for titans). Grind strikes for blues and greens on your new class would be my advice.

Make the titan. I think the game was designed to have people max out all 3 classes (and subclasses ). The game takes on a whole new life once you do. First off there are always opportunities to raid. Its actually pretty fun playing through the game again and learning new skills. You probably have some titan gear collecting dust in your vault. If you add these to the blue engrams that you save until you're 20, youll probably be at least 26 immediately after equiiping once you hit 20. You have a steady stream of bounties to upgrade weapons. The list goes on. But, most importantly for me, its just really fun using all of the skills. If im using hunter, there's times when i feel like "damn, i feel like shoulder charging something. " or on my titan, ill be like "Damn, i feel like throwing big ass niva bombs." Start the titan.

Do it, I deleted my second titan last week to make a warlock everyone should have one of each class, but I wouldn't just leave him as a defender all the time striker is a great class and those lightning grenades are sick.

Titans are awesome dude. Play one

having one character from each class not only lets yo incorporate different play styles into your game, but once each character is leveled how you want, you are ready to take on any task. say you are going into a Strike or Raid and there are more Warlocks and Hunters. you could bring your Titan to provide defense. or say that there are more Titans and Warlocks. your Hunter could provide sniper cover while sneaking in to do extra damage on bosses with their Invisibility Cloak. it really just depends on how you play, but from the beginning I created one of each class for just this purpose. it IS a bitch leveling up all three characters, though. keep in mind the grind time you will need to maximize a new class.

Do it.