skip levels are useless without multi level, but since you probably shouldnt use your ML till the last levels, you can either keep them or play them, doesn't really matter either way (as long as you keep the same amount of skips as you have ML)
They can still be useful without an ML to jump to later levels to get to the longer levels with more AP returns - if that's what you're down for. If you prefer to take on the shorter levels in order, then they serve no purpose.
I've used them to skip to the highest levels and clear them in reverse order because I find that the AP per time invested on those levels is higher.
(This was even truer when I first started doing this and the levels were less balanced, and I was able to finish the United States in less than 2 days, albeit using a bunch of Time Warps, Caches, Market Raids & Inspire Mercs.)