I have the 8120 and never use the wifi, there just never seems to be a reason, my sister has a new curve with the GPS and I had a play and it's super useful. Much easier to use than google maps which is my only option with the pearl as I don't want to bulk it up with an extra dongle.
The Freedom Keychain 2000 isn't exactly what you'd call a bulky dongle though. I personally find the wifi option quite useful, speeds up internet browsing and now that Opera Mini can use wifi it's a definite plus imho
8120 makes more sense to me.
Useful if you do not have unlimited data plan.
Useful to avoid roaming data charges (just use your hotel's or coffee shop's wifi).
Allows you to use services like Skype (pay 2 cents per minute rather than roaming and/or long distance charges).
I use a external bluetooth GPS the rare times I need to use GPS on my phone. I would never substitute my automobile's GPS for one on a phone.