Just what good would a factory image be, you ask? Well, first of all, it will enable one to reset to stock should you actually go through a rather unpleasant experience while rooting and installing a custom ROM on the Nexus 9. Do take note, however, that custom ROMs for the Nexus 9 would be some ways off since the sole availability of the source code is not adequate to come up with an actual, working ROM. One would still need to wait on when it comes to the driver binaries, which does seem to take its own sweet time, it seems.
The Nexus 9 has already been taken apart by the folks over at iFixit just in case you are wondering, and its innards are impressive to look at, although it can be a pain to repair by yourself if things go wrong.