Here is what I found regarding the issue if chickens need oyster shells or not. Couple different websites.
http://poultrykeeper.com/keeping-chickens-faq/why-do-chickens-need-grit-oystershell
Soluble Grit
"Soluble grit (the most common being Oyster shell grit) often gets confused with flint or insoluble grit. This kind of grit is digested by the hen and contains calcium, most of which is used to form strong egg shells. The majority of an egg shell is made up of calcium carbonate. A constant supply of oyster shell grit ensures they have sufficient calcium in their bodies. If insufficient calcium is available, they can lay soft or poor quality egg shells."
http://www.extension.org/pages/69065/feeding-chickens-for-egg-production#.VL52AIFHarU
"Laying hens require large amounts of calcium for eggshells. Laying mashes typically contain 2.5% to 3.5% calcium. Growing chickens require only 1.2% calcium in their feed. If you feed high-calcium diets to growing chickens, kidney damage can result. It may also be necessary to supplement the diet of laying hens with ground oyster shell on a free-choice basis. Some high-producing laying hens may require the extra calcium that the oyster shell provides. Monitor the quality of eggshells to determine whether or not you need supplemental oyster shell. If hens produce eggs with thin shells or shells that are easily cracked, oyster shell supplementation might help.
Layer diets should contain at least 14% protein to ensure continued egg production. Layer diets that contain 16% protein are more common."
http://www.merckmanuals.com/vet/pou..._poultry/mineral_deficiencies_in_poultry.html
"Diets must provide adequate quantities of calcium and phosphorus to prevent deficiencies. However, feeding diets that contain >2.5% calcium during the growing period produces a high incidence of nephrosis, visceral gout, calcium urate deposits in the ureters, and sometimes high mortality, especially in the presence of infectious bronchitis virus. Eggshell strength and bone strength can both be improved by feeding ∼50% of the dietary calcium supplement in the form of coarse limestone, with the remaining half as fine particle limestone. Offering the coarse supplement permits the birds to satisfy their requirements when they need it most, or allows the coarse material to be retained in the gizzard where the calcium can be absorbed continuously. A readily assimilable calcium and/or calcium phosphate supplement is effective if started very soon after paralysis due to calcium deficiency develops."
I guess I shouldn't have said the words crucial....yes most layer feeds contain calcium. I have found when oyster shell is not offered we do get a reduced number of eggs and they tend to be thinner shelled. Of course do what you feel is best for your birds.