Black ink changes.....this is purely a guess....however, I suspect there is a good reason to switch the blacks.
For example, if I do a lot of printing with papers using PK ink and little matte printing using MK, the MK line and nozzles will get little to no use for and extended period of time. When I want to use MK, I would expect the nozzles would surely be dried out and clogged solid.
I read a report somewhere, not sure which forum, of someone who had a problem like this. After only printing using PK they eventually did a switch to MK and got nothing from that channel. So they switched back to PK which had been working well, and got nothing either. As I recall, it turned out that there was a lot of sedimentation around the ink selector switch, and it took quite some flushing to clear this. I *think* this was OEM rather than 3rd party. This was only one instance and perhaps they were unlucky. I print on both kinds of papers and see the switching involved as a forum of insurance against this, unlikely as it may be. (The nozzles in the head should be unaffected, as PK and MK use the same ones.)
As Mark indicated in his review, only months of use by many consumers will provide the empirical data needed to determine under what circumstances the P800 might clog. But given that its basic head design is taken from the 3800/3880 - easily the most reliable pigment printer Epson has ever sold - my guess is that the P800 will continue that steadfast tradition.
I fully agree that only time will answer this question conclusively. But there are some people still able to source a 3880 and who are undecided which way to jump. There are several P800s out there in forum members' hands now. Has anyone been able to do a direct comparison to see how similar they look internally and how they seem to behave? Might give us a little more to go on regarding the likely behavior of the P800, other than the apparently similarity of the head.