sure, I wouldnt for a heartbeat knock back a hub dynamo , but building a wheel? if you can do it yourself fine, but for the average Joe(if you can work an allen key bb is easy to install), not going to get much change out of $300(more than my ride is worth) once you go through the whole process, more if you buy the high end, and of course if you swap bikes then the wheel may not come across, or someone stealing the whole wheel!

and if you ride the bike for exercise, the extra drag from the bb dynamo is just part of the fun!, and of course there is no increase in rotational inertia andtranslational inertia of front wheel or drag when it is not being used.