Why do modern PP OPTs have 16 Ohm taps?

[PGDO]

The (comparatively) more expensive Avantgarde horn speakers models have a 18 ohm impedance.

 

[tinitus]

I have seen a 32ohm Eminence guitar speaker(custom)

 

[SY]

useful if you have speakers with bi-amp terminals and you want either section to be +3db from the other.

Exactly right. I did this with my last MTM speakers, wiring the mid/woofers in series. Worked beautifully.

One other bonus- a 16 ohm winding with a 4 ohm tap is perfect for implementing cathode feedback in the output stage.

 

[tinitus]

One other bonus- a 16 ohm winding with a 4 ohm tap is perfect for implementing cathode feedback in the output stage.

interesting

 

[JMFahey]

I have seen a 32ohm Eminence guitar speaker(custom)

You're right, but even more, you needn't order it custom made , it's in everyday production , available at any Speaker dealer:
Eminence Legend B810 10" 32 Ohm Bass Speakers - Eminence B810 Legend 10" Speaker - Eminence Legend B810 is a 32 ohm 10" bass guitar speaker. The Eminence Legend B810 is a great upgrade or replacement 10" bass speaker.

Not only that, the ad clearly states:
AVAILABLE IN 32 OHMS ONLY