Don't get out much do ya? Probably not many small time flower planter/lawn sprayer type landscapers with a disc, but ANY landscaper worth their salt that are capable of doing larger jobs will have one. They are the absolute BEST way to prep rough areas for finishing. Tillers generally aren't the hot set-up around new construction, highway right-of-way landscaping or any place where rocks and/or construction debris will be frequent. Tilling isn't a great way to prep soil for grass seeding under normal circumstances. Grass doesn't like a deep, fluffy seedbed. A neighbor to my east "farms" almost 1000 acres, growing turf....sod.... sold to builders and golf course construction. After stripping sod off a parcel, he ALWAYS works the ground with a disc, then seeds with a Brillion seeder.
no construction companies with disc's either.. just farmers there haus... i've never seen a disc on a residential construction site... graders/dozers/loaders/bobcats.. never a disc.
for me a disc is after the plow. that's if you're old school or can't do no-till in your soil.. go find me an authoritative site, maybe an extension office that recommends discing for overseeding, i'd be interested in reading it..