First reason is that my normal setup is gut mains/17 or 18g copoly crosses. Also, unlike some posters, I'm attracted to the low stiffness and power of ZX. I'm looking for a stringbed as powerful as gut but at a lower price, and hopefully with more spin and durability. So the 17g Pro version of ZX seems to fit my goals, as thinner strings are less stiff and more powerful at a given tension than thicker strings are.
Additionally, I'm obsessed with lateral main string sliding and snapback, for three reasons:
1. When the mains slide laterally during impact, dwell time increases and shock decreases. Even though gut mains/ZX crosses should be among the softest string combinations possible, more dwell and less shock is still welcome.
2. If the mains snap back to their original positions quickly enough, while the ball is still on the strings, they can put additional spin on the ball. I'm hoping that gut/ZX will offer something like the spin potential of gut/copoly.
3. Straightening strings is so 20th century!
Now, in theory, thinner cross strings should allow the mains to slide and snapback laterally more easily, because the angle of the weave is less and therefore interstring frictional forces should also be lower. If the crosses were microscopically thin, the mains would be completely free to slide, and you'd effectively have a neo-spaghetti stringjob. Not sure how big of a difference 17g vs. 16g makes, though.
On the other hand, JackLondon and I have been discussing an alternative POV - that flexibility in the cross string is not good for lateral main string movement. A more flexible string will deform locally, forming a "hill" in front of the sliding main string that it must continually climb. The TW Professor wrote about this a couple years ago. From this perspective, the stiffer and more rigid the cross string, the better the main string should slide along it, kind of like sliding on rails rather than along a rope. Now, one of the unique characteristics of ZX is that it is very flexible longitudinally, but is not pliable locally like a nylon string is. So it may be better as a cross than even a slippery nylon string would be, even though ZX is longitudinally less stiff than any nylon string. Even still, one would think that a thicker gauge ZX cross would be more rigid locally and serve better as a "rail." So I may be making a mistake in going with 17g Pro.
As for tension, in this thread
I posted deflection numbers for natural gut, ZX 16 and a copoly that I often use as a cross with gut mains, Proline II. I typically string gut/Proline II at 52/50. Comparing the deflection numbers, it looks like gut/ZX 16 at 57 might be about right to replicate somewhat the stringbed dynamics of gut/PLII at 51. Torres' recent experiment with gut/ZX suggests that 57 might indeed be a good starting point, but with the thinner Pro maybe I should go with 60. It will probably be a gametime decision.
Longwinded post, as usual with me, but there it is...