I discovered on my Canon dSLR that its stopped-down metering with lenses from film cameras was flawed in its accuracy! Proven via multiple tests with multiple lenses.
What do you mean by fast handling?
With a 500 ELM (motor drive), all you need to do is aim and focus. To shoot and advance the film, you just press the release button. Seems pretty fast to me.
Use the Sunny 16 method and you don't even have to meter the scene.
The Pentax 645 lenses are not that expensive (mostly) and work much better with the body than adapter Hasselblad lenses, so there is not a lot of impetus to do this unless you already have some lenses. But I did find it useful for lenses that I did not have for the Pentax - a 30mm fisheye, and an Imagon.