If its the type of music that works with programmed beats and loops (as much pop, and even rock, music these days can), or if it is the kind of music that doesn't need drums at all (like if it is very "folksy"), then, no, drums aren't essential. If it is the kind of music where you really want a drummer, but are trying to get by with a "drummer substitute", then yes, real drums are probably essential. Tracking drums later is maybe not the best solution, but it can work. Better yet, can you just hire a drummer for the sessions (and some pre-production)? Find a good drummer in your area who plays in a band similar to your style and see if he'd want to do a short side-gig for a little bit of cash. I've had good luck with that. If he's into your tunes, you may not even have to pay. He might just do it for the recording credit.