Message Requests are currently used primarily when unencrypted mails (specifically those not made using QRCode/Link) on regular mail addresses. But especially for those which may have their links public, always having Message Requests can be very useful, or even important.
I’m not sure if Delta currently prevents sending read receipts before accepting a request, but that is sort of a requirement for this to work; Without it there is little to no benefit to doing this. Also not sending bio, avatar etc until a request is accepted would be useful.
I’d very much like to see this as a security/safety measure
For reference, Signal currently does this, including showing a preview of the message but not sending read receipts.