I'm guessing, but maybe there are strategic reasons as well. So that it is more important to keep certain countries as allies, even if they don't fully pay their way, at the moment.
Consider an ex-USSR, Baltic state country. Typically they have little money/resources.
So they are of important military advantage to the US. Because it means that the US, can place military items (e.g. Anti-Nuclear bomb, ballistic missile, defense systems, potentially in countries near Russia). Early warning radar systems, air-bases, sea port access, military bases etc.
There is probably important (military secrets and open stuff) information, coming from countries, previously tied in with Russia, and physically near. Many of the population even speak Russian.
So if the US considers Russia as a potentially significant and possible future (or even current) enemy. It makes sense to keep those countries on-board, as it may save the US a lot of time, expense and hassle, if it has to cope without all those various advantages.
Analogy:
(Not connected to Russia).
If South Korea and the US have good relationships, and "crazy" North Korea decides to start taking pot shots at US assets, via North Korean Nuclear Ballistic missile(s). The US can just press a button, and shoot them down via anti-ballistic missiles based in South Korea.
But if the US did not have South Korea as an ally, the US might have to put up with North Korean Nuclear Bomb armed, Ballistic Missile(s), taking pot shots at US territories.
It's possible the missile defense system is only for South Korea, I'm NOT sure. But anyway the high performance radar systems, help other systems track and take out enemy ballistic missiles, so it still is probably useful, anyway.
Most jet fighters only have a limited (easy, without re-fueling) range of a few hundred miles or so. So having military bases all over the planet, and many aircraft carriers, helps keep the US (and NATO) well defended, wherever in the world, enemies spring up.
If Trump were to tighten his belt, and turn many of the currently available military bases, and friendly/ally countries, into unfriendly/unusable entities.
Then fighting ISIS, and any other possible future enemies would be much harder, and possibly considerably more expensive.
tl;dr
I think it would be a false economy to mess with Nato too much. It might even leave the US less defended in the future.