Why are cab units preferred for passenger service?

[Puddlejumper]

RS-3s only had a high short hood (hammerhead) if they had Dynamic Brakes and Steam Generator. The SG fit in the low short hood if no DB were present.

Good point. I'd wondered why the hammerheads existed, since I remember seeing normal RS-3s with steam generator exhausts on the short hood.

I seem to remember that even some RSD-5s had SGs - for passenger trains on lightly laid branches?

Yes, and I believe CNW had some Hammerhead RSD-5s as well.