My Hassy kit came with a metered prism finder that I don't use. If you have a 503cx or similar model, I believe the meter works in conjunction with a Hasselblad (or other?) flash for automatic exposure control. The guy that sold it to me used it for flash wedding photography. I just use it as-is.
The metering electronics inside the prism are not linked to anything else.
TTL-flash metering is achieved by hooking a sensor inside some Hasselblad camera bodies, through the appropriate adapter, up to the electronics inside suitable flash units.
The Hasselblad meter prisms are very easy to use, and dead accurate. If you want to use a prism anyway, no reason (except perhaps financial ones) not to make it a metered prism.
Hand held meters are very fine tools. But there is no reason why you couldn't get the same excellent results using a built-in meter. The biggest contributing factor to getting good results is you knowing what you are doing, not the meter, or what type it is, itself.
So in the end, it is a matter of personal preferences. Some people like spot meters, other people incident metering. To each his own. They all work