I think the point of using more mains is that control improves. The most recent research on spin from TW University showed that the 16x15 patterns in the two Steams both generated more spin than conventional patterns, but it also revealed that when the main strings move a lot, as they do with so few crosses, a gap is created in the string pattern. Here's a link to a photo of this gap:
http://www.google.com/imgres?imgurl...a=X&ei=VORiUo6JLcWcyQHcyYDQBA&ved=0CDkQ9QEwAg
This gap has two undesirable effects:
1. Depending on precisely where the ball impacts relative to the strings (e.g. is it contacting three mains squarely, or spread along four mains?) the rebound angle of the ball will be different. Basically, the gap between the strings results in a very high rebound angle, and with few mains this angle tends to be quite different from shot to shot. So if you hit the "exact same" shot twice, but the ball impacts the stringbed in a slightly different position each time, the rebound angle will be quite different on each shot. This has always been a problem with open patterns, and it seems that when the mains are free to slide sideways this problem gets worse.
2. This gap in the strings actually produces a torque that is in the opposite direction of topspin generation. So the main strings slide sideways and then snapback, which produces more spin. That's the entire point of the Spin Effect technology. But when relatively few mains slide sideways quite far, as they do in these frames, the gap that is created between the sliding strings and the non-moving ones allows part of the ball to squeeze into it. That part of the ball receives this counterspin torque, a force that the TW Professor calls a "normal force offset." Again, depending on where the ball impacts relative to the mains, this gap can be larger or smaller. If the gap is large, spin will be less; if the gap is smaller, spin will be greater. So this variability in the size of the gap results in variability in how much spin you get from shot to shot.
Taken together, variability in rebound angle and variability in spin results in wide variability from shot to shot that could be called poor control.
The point of increasing the number of main strings is that, with fewer crosses, those mains can still slide sideways, and so you still get the snapback spin effect. But with more mains, the relationship between the ball and the main strings will be more homogeneous - with 18 mains you have a better chance of the ball spanning six mains each time you hit it, whereas with 16 mains it might span four mains one time, five times the next, etc. Basically, the more mains you have on the ball the smaller will be this gap that appears when the mains slide sideways, and the smaller that gap the more consistent your shots will be, in terms of both rebound angle and rpms.
My sense is that Wilson is trying a few different patterns in different racquets. They probably have a feel already for how each will perform, and what type of player might benefit from each. For example, the faster you swing and the higher the swingweight of your frame, the more your mains will slide. So, a pro player swinging 80 mph on the forehand side will have no problem getting his copoly strings to slide and snapback, even with a conventional 16x19 pattern, especially if his racquet has a 350+ swingweight. But a rec player, swinging at only 50 mph, might not get those same strings to bend and slide, let alone snapback - and so he won't benefit from the slippery copoly strings. But give that same rec player a 16x15 pattern and, because of the reduced friction at the string intersections, he'll start to experience sliding and snapback and the extra spin it brings to the table. So, for higher level players, 18 mains might be great - the strings won't slide quite as easily as they do with a 16x15, but because he's swinging fast he doesn't need them to. And those extra mains, as explained above, will give him more control in terms of less shot to shot variability.
This is all quite complicated, and gets even worse when you consider the many different copoly strings on the market (which vary widely in both stiffness and surface friction) and the various tensions people string them at.
BTW, the study I mentioned above is well worth taking the time to read. TW University is down today, but I'll link to it later in this post. My guess is that, once all the Spin Effect and Prince ESP racquets hit the market, TWU will do some similar work comparing all those racquets in terms of spin, rebound angle, shot variability, power potential, etc. That information will be very revealing.