Converting to CMYK and soft proofing in CMYK are two completely different things, so at least we need a CMYK softproofing.
During the last years I ran into a lot of problems due to modern LED lightning on the scene, it is able to produce colors (especially in the blues) far away out of the CMYK color space. Converting pictures to CMYK later after my initial processing can change a picture dramatically, a bit similar to a conversion to b/w.
So even if you have to deliver your material in RGB and someone else is doing a professional conversion later, you should be able to predict what can happen to your material!
So if you deliver material that will be converted – earlier or later – to CMYK: You need at least a simple soft proof. Otherwise it can happen that you deliver material that simply can’t be published.