Locking a line down would basically guarantee you always get 2-3 lines. You'd just sit there rerolling at 7 until you get 2 intersecting lines before locking down the third line that runs between the two of them.

Only reason to not always go for 3 lines is if it's the end of the week and you're in a rush to finish up but I'm assuming when the odds for 3 are good most people wouldn't mind waiting for it even if it cuts into their next WT.