It depends on which rack you talk about.
Some of them are made with things really dedicated to only one effect. Like you would use a 'reverb' rack unit, then a distortion, then a delay etc... In that case, yes, they are worth it, since every link of your sound goes into the best thing there is!
You also have some rack units that actually are multi effects, and then, the sound depends on the quality of your multi effects (like if you use an eventide harmonizer, that costs around 10000 bucks, it might sound better than anything else) And yes, it's quite more complicated to use a rack than a pedal, but depending on the sound you like, you can use either of them.
Some people use a rack because they need to get all those different sounds from all their albums others can achieve that with stompboxes (like satch). Some guys (like me)have had racks that were as big as a fridge, and then went back to 'a guitar, a cord, and you good ol' fingaz'.