16bit DAC with 24bit SPI?
You are making too much of a minor inconvenience.

First, you probably can do two 16 bit transfers.  The device will respond with zeros or garbage in the extra 8 bits, and you ignore them in your firmware.

However, the logical thing to do is three 8-bit transfers.  Surely your micro can be set up to transfer 8-bit chunks.

At 100 kS/s, you have 10 &micro;s per sample.  That's a "long" time for SPI that can probably be clocked at 10 MHz (I didn't check the datasheet, but such speed is usually supported by such devices).  Each bit therefore takes 100 ns.  Transferring 24 bits therefore takes 2.4 &micro;s just for the bits, plus a little overhead to select and de-select the chip.

Overall, you should be able to transfer the necessary data in about &frac14; the available time, certainly within &frac13; the available time.

There is no problem here.  You just have to architect the firmware up front, taking the specific characteristics of this D/A into account.

As for why the manufacturers do this?  Two whole bytes are used for the data.  You said yourself that some status and configuration is also transmitted, so that obviously takes at least a part of one more byte.  Most SPI masters have hardware that transfers whole bytes, so they document the extra bits as being in a whole byte.  They correctly realize this is really no big deal to firmware writers.