15 plays equates to about 1 quarter of football.
Walsh scripted them because he would go thru all of the personnel packages and formations he planned on using and would see how the defense would try and defend those personnel groups and formations. Then in the 2nd, 3rd and 4th quarters he could then adjust what plays from those formations/personnel groups would work against the way the defense tried to defend it.
Works great for somebody like Walsh, but I think with many O-Coordinators their brain doesn't work that way. Also, you need some good talent because if you are stuck on those plays and they don't work and the opponent gets out to a big, early lead...the game can be lost right there.
YR