mobile broadband vs tethering, any benefits to dongles e.t.c

My phone supports tethering, so what's the point of getting mobile broadband (i.e. usb dongle e.t.c.)

I don't see one. Surely I should just turn bluetooth on and surf the web using my phone's connection.

I don't believe a dongle will get a better signal.

Am I wrong?

The only advantage I can see is the possible increased latency and decreased by using your mobile phone with tethering, especially if you are tethering over Bluetooth.

On the other hand, a USB dongle is designed to be used with a computer (so it should have a lower latency) and speeds would be faster because of the USB connection.

I don't see a point in getting a second device just to connect to a laptop. My G1 (with Cyanogen's rom) tethers quite nicely with USB, bluetooth and wifi. The extra latency and power consumption are outweighed by the ridiculous prices of data only plans here in the United States. I can obtain fairly good download speeds on T-mobile's HSPA network.