At a recent conference with Jim Balsillie as the speaker, he mentioned that the current GSM carrier infrastructures could only handle so much data before caving to the demands of 3G technology. Current UMTS/HSDPA/HSUPA networks are VERY limited in allowed bandwidth from all 3G-capable carriers (in North America). Needless to say, there is a lot of truth in the fact that there is a high demand for a technology that will not necessarily have the infrastructure in place to support it at full capacity and full capability. If AT&T or the like would offer full speeds to everyone, I'd be very surprised.
As for the benefits, 3G can really come in handy when you're streaming video over the network, or downloading raw attachments from messages (OS 4.3), or remote upgrading device OS (OS 4.3+BES 4.1.5), or a lot of other possibilities that are being added. There are also a lot of internal application development items at a lot of companies that could benefit from 3G technology and speed. RIM can see the necessity and benefits, but I think they're also looking at it from the aspect that they have a very popular device and platform and could end up having a negative experience when it is implemented.
Regardless, we'll see it sooner than later.