As an item I'm not really a fan of Force of Nature. The cost is extremely high and you get two things that don't synergize that well: a massive amount of MR and a pretty underwhelming of health regen. There are certain situations where it makes sense to consider, but I never really find it fits my playstyle well. I don't tend to feel like it makes tanks threatening enough for the gold investment/item slot, but this might just be more of a personal preference.
Not a fan of FON normally either, but I find the movespeed to be absolutely crucial with Skarner. The way I look at it, movespeed is an indirect buff to Skarner's ultimate - the more distance you can cover when the target is suppressed, the further you'll be able to pull the enemy carry away from your team or the harder you'll be able to pull the divers off your carry.
Plus FON provides you the single biggest chunk of MR of any item in the game. That plus Merc Treads and you should be good for MR.
But yea, I only use FON on champions that I believe need the movespeed, like Singed or Skarner.