My point of view about this not being a great feature is that even if you already own a (good) power supply, either static or batteries, you still can’t power the Gadgeteer boards without an USB Client module or at least an extender module that will do the interface between your power supply and the board.
Also, if you want your board to be standalone, say on a robot, you would absolutely need the USP Client DP module because it has a power plug and not the USB Client SP, which is using an USB connector.
Lastly, but correct me if I’m wrong, I’m not sure that the USB Client SP module would help providing enough current with many devices connected to the board. Remember that here you only have max 500mA available.
In all case, you could use an extender module but will need some work on it to connect it to your power supply.
If you build your own power module, this will also add extra cost and work designing and soldering it.
All in all, this increases the final cost in a way you can’t avoid without requiring extra work from you . This is why I don’t think it’s a great feature, but only a feature.
But, as I said, it is by design, so it surely has been discussed seriously and good reasons exist. That’s why I say that we can’t say much about it now.