A more open string pattern, i.e. "less string", does indeed create more spin. But, eddleng, you have consider the uneven spacing of the strings when you skip holes. Although some rackets does have slightly uneven string spacing (e.g. KSix-One 95 16x18 has a more open pattern near the frame edge than the centre of the stringbed), making your own uneven spacing may cause the tennis ball to fly all over the place, or not fly at all. Besides, there's the problem in stringing. First, it's impossible to have an odd number of mains strings, so 15 is out of the question. Also, when you decide to skip holes, I'm sure you (or the stringer, if he/she is willing to do this crazy experiment for you) will have a difficult time to count the holes.