FAR 37.602 addresses using a Statement of Objectives (SOO) instead of a SOW in a solicitation. With a SOO, the agency puts the SOO in the solicitation that simply spells out what results/objectives the agency is seeking. Then the offeror submits a proposal, and if selected for award, that proposal gets adopted into the contract as the SOW/PWS.
In real-life practice, how would the agency craft the evaluation criteria for this? Let me illustrate the problem. If the agency does a LPTA, how can the agency determine whether a proposal is technically acceptable if there are no "requirements" in the SOW, b/c there is no SOW, just a SOO? Remember, FAR 37.6 states that the SOO never gets incorporated into the contract. The SOO does not provide "requirements," only "objectives." The contractor's proposal becomes the PWS with the requirements.
If the agency does a TRADEOFF, how does the SSA realistically determine if one proposal is technically superior to another? Once again, we have no "requirements" in the original solicitation. In this scenario, let's assume that the evaluation criteria are TECHNICAL APPROACH and PRICE.