One of the Cura Dev's here.
Cura does support USB printing. It's not tested very well on different machines. As already mentioned in this topic, the developers paid by Ultimaker won't spend tons of time on making other printers work (the fact that we added USB printing in the first place is already a bonus as 99% of the UM users don't use USB printing). It could well be that the printr bot does some things slightly different from the Ultimaker, rendering it unusable.
The USB printing is different from the old cura, as it now actually checks if there are suitable printers before it shows the print to USB button. If you check the log files, you might be able to see some errors that point you towards why the printer is not detected. I've copied a ton of code from the old cura for the USB printing, so it should behave in the same way.
If a developer from printr bot wants to improve on the USB printing they are free to do so. I could make a bit of time to help them out with questions if they have them. If printer bot wants Ultimaker to build these features, they could also drop me a line. I'm sure something can be worked out here.
We're probably not going to implement a full custom designer for profiles as improving on the site that quilford made is much easier and does exactly the same thing (and again; We have finite resources. If we have to choose between improving Cura (or making the experience for the UM better) and adding some feature for other printers, we will choose to make it better for Ultimakers (Because ya know, they still ensure I have a house to live in and food to eat
). And once more because we can't stress on this enough; Cura will stay open source just the way it is.