> I'm guessing that the purpose of this is to be a "super-lightweight cups" for the common case where you want to print (a) infrequently and (b) to a fairly standard page printer (ie: not a receipt printer or anything).
I don't understand the point of writing less capable software just so that when somebody needs to be able to do what is relatively easily done in any other OS they must rip out your software and/or reconfigure their OS.
> It will presumably be able to talk to IPP servers (via the relevant cups driver?),
Typical 'cups driver' is just a postscript description file.
Postscript is a programming language that is used to instruct the printer on how you want you page printed. Cups will take the postscript from your application and then insert additional instructions to it based on the PPD file (aka 'driver') and your configuration dialogs. Instructions on how to handle ink, line speed, stapling, duplex, etc etc.
This is why you need things like Ghostscript, because CUPS must have the ability to tear down postscript 'documents' and then rebuild them for specific printer models.
Then, of course, there are a large number of printers that do not support postscript, proprietary psuedo-ascii line/serial printers, or printing tasks that require other languages like PCL.
On top of all that modern printers are designed to be really stupid. They tend to no longer understand proper printing languages. Instead they are little more then over-glorified USB accessories were you have a proprietary (as in printer-specific) daemon that is able to take the postscript/pcl/etc instructions and transform them to the device-specific instructions necessary to operate the printer head and emulate a traditional printer device.
All and all even relatively 'basic' printing tasks tend to be fiendishly complicated and require a heavyweight solution.
So unless printerd can replace CUPS in it's entirety there is no point in having it do anything except just being a spool manager that replaces some fraction of CUPS functionality.