Why?

You are going to laugh but I have to ask: On my iMac with FCP7 and a ProRes HD timeline I can plug an HDMI monitor into the spare thunderbolt/DVI port and edit away fine (im just editing , grading and captioning ...do I actually need an ultra studio mini monitor (or some other) device ...would there be any benefit in my setup?
thanks

The DisplayPort is outputting sRGB in Apple display setting gamma. It's not a broadcast signal.

The Mini monitor is for monitoring a broadcast signal (like Rec.709) on an HD display or Calibrated Broadcast Reference monitor.

All depends on your final delivery requirements. If you are delivering to the web only you probably don't need the mini monitor.

If you are mastering for broadcast, digital cinema release, DVD, etc then you will benefit from proper monitoring.

Thanks for the info Adam.
If I understood correctly; the MiniMon's 'job description' is realtime colorspace translation.

If so that might fix my dark contrasty images on large oldish LCD screen I use , but will my After effects projects (currently set to rec709) need default sRGB reset, to avoid the color being interpreted a second time ?

Correct me if I'm wrong, but I'm pretty sure you're not going to be seeing any fields with your vision either. So if you're working with interlaced footage and you happen to swap the fields or there is something funky going on there, you're not going to know about it.

But as was mentioned above, if you're not going into a broadcast environment, it's probably not going to matter that much.