I don't know if I would be correct to assume this, but I'm thinking that crosses provide at least half of what we conventionally consider to be tension. For example: If I string a frame at 55lbs but stop to observe the tension once the mains are strung, I would find that the mains aren't at a tension of 55lbs. In fact they are less, only after I add the crosses will the tension read 55lbs. Thinking this, I would reason that the crosses are in place to restrict the mains.
If this were my reasoning behind string tension, then in the event that poly mains break poly crosses, I would be able to assume that since the mains and crosses are acting as two seperate forces in the string bed, ultimately one may force the other to break. Its possible that a poly cross could break if poly mains saw through them, but most of the time the mains will snap or break because the same main has multiple points of contact and restrictive force.
The main must either notch the cross; or wear thru the cross, i.e. shaped poly with point in contact with flat surface, force is concentrated at the point. How this happens is mysterious to me unless it is a byproduct of a WW swing shape? I can see this happening when the mains are made of material harder than the cross as in a hybrid, but not so much when the main and cross are the same string.
i always read about mains notching into the crosses and see that one important aspect of the wear on the crosses is mostly overlooked - that is string to ball friction. this friction, with me as a heavily topspincentered player, wears down crosses, and i'm talking here poly crosses, far more than the friction that comes from the mains gliding across the crosses on each shot. this is easy to observe if you look top down on your stick and see how crosses are thinned out. the fact that the crosses are mostly thinned out exactly on the intersections with the mains results in the fact that obviously the string to ball friction has a rather important role in wearing down the crosses. the area which is mainly exposed to string-to-string friction is thicker, meaning that there less abrasion occurs.
from my findings so far i may conclude that if having a full bed with the same string (same diameter), usually i break mains first. if i go with the same string but a one gauge thinner diameter on the crosses, the crosses will usually break first but the mains would be on their last shots too. if i go with a softer cross but same diameter as the mains it usually is again the cross going first. on the other hand a softer mains would basically go in no time, leaving the cross in almost pristine condition.
I string for a guy that always broke the main only. He switched to Gamma Zo Verve main and Forten nylon crosses. He then started wearing the Forten ribbon thin before they broke. The Forten string was flatter on the side that contacted the ball rather than the side touching the main. The mains had no groves and very little wear.
I string for a couple of guys who break crosses. The crosses wear in patches of about half a centimetre either side of where the mains are. It looks as if someone has taken a file and roughly sanded them down in a back-and-forth fashion. Eventually, it reaches a point where the string is thin enough to break.
The only real trend between the two players is that they both like to hit a big ball. One strings at quite a high tension, the other likes it very low. Ability wise there's a fair gap: one of them is a coach who used to play on the futures circuit. His technique is pretty much flawless. The other guy is good, but not on that level.