First, you DON'T need the Hotspot plan in order to use wifi. You can browse the internet with standalone wifi, even without a sim card installed. Some applications can access the wifi connection and use it for data but not many have been written this way yet.
Second, you DON'T need the Hotspot plan in order to use wifi/UMA. I don't subscribe to the Hotspot plan and I use UMA all the time. Minutes used over wifi just come out of your minute bucket (rate plan.) If you add the $10 Hotspot plan you get unlimited voice minutes over wifi/UMA.
Third, you DO need the proprietary Blackberry Data Plan ($20) if you want email and other Blackberry services. This service goes through the RIM data center and you have to pay (i.e subscribe) to the service in order to get full features out of a Blackberry (any Blackberry, including wifi BB's.) Wifi just provides a faster conduit to connect to the service as opposed to EDGE network..