I truly believe that pull ups are an essential component in any program. So much so that I had my own custom pull up bar built. It's a stainless steel beauty and I'm LOVIN' it!
But you don't need a fancy rig. The main thing is to be pulling your body up while hanging on to something. My cousin's husband has a couple pieces of rope with tennis balls knotted onto the end of them suspended from a mezanine railing. He just hangs onto those and does pull ups alternated with other bodyweight exercise variations and he stays in great shape.
No one told him he HAD to do pull ups. He was just instinctively drawn to them. And to me, that makes perfect sense.
Our ancestors would have been in a pretty tight spot if they weren't able to climb a tree to get away from danger or pull themselves up a rock face to access the safety of caves and protected ledges. We were "made" to climb.
Unfortunately, pull ups is about the most ignored exercise in the modern gym goer's repertoire. Bench presses and squats are much more sexy. But ignoring the flip side is both silly and potentially dangerous.
Even if all you want is bigger pecs. You still need to be do tons of pulling movements to keep your shoulder joint in balance and continue pushing hard in the "mirror" lifts. If you stop working your back, your brain will shut off your pecs too before you can get too far pit of whack. And if it doesn't... there goes your shoulder.
So find something solid to pull yourself up and, and start climbin'...