Some pellets can be good, especially if you don't have a wide variety of leafy greens and herbs to offer on a regular basis to make up for the vitamins and minerals they normally get from pellets. The key is to feed them in moderation (and a pellet free diet IS a viable option, it just requires a little more effort when it comes to quantity, type and rotation of veggies). Usually, it's about 1/4 c per 6 lbs body weight per day for adults. Juvenile (under around 7 mos for most breeds/12 mos for large breeds) rabbits can technically have unlimited pellets, but most people find their rabbits do better and eat hay better (especially in the long run) if they somewhat limit the pellets for their young rabbit. I tend to recommend a maximum of 2x what their adult ration of pellets will be (based on their expected adult weight). Another good option is to hand-feed pellets as treats, which also helps them bond with you.
Think of it this way - if you had a kid, you'd probably give that kid one of those Flinstones chewable vitamins or something similar, right? But you certainly wouldn't let them eat half the bottle in one day. Pellets are the same way - they help bridge minor nutritional gaps just to make sure all the important vitamins and minerals get into your bunny... and like vitamins, they're a supplement rather than a meal.