hellflower is always your first offensive item since it will basically allow you to jump anyone and kill him
then after hellflower you need surviveability and more mana regen so that makes nullstone an excellent item
and well if enemy team got some big target spells like hammerstorm or maybe voodoo jester nullstone is awesome since wasting a huge stun like that just to proc a nullstone can win the fight

But there are almost always stuns and snares for single targets... So it's almost ALWAYS a must, especially past like 1500.

Ya I'm just saying since earlier game (for me anyways at 1600) is full of 2-3 player ganks and not a lot of huge team fights, it can be put off if your not trying to counter very powerful early gamers like pebs, dw, sb ect.

Since the introduction of the pretty much constantly available matchmaking, the average rating of the players has dropped. It used to be around 1600-1700, now it's back to the old 1500-1600, and I think at around 1600 is when you should consider starting to think in terms of higher-end gaming mindsets. Null Stone, I would say, is necessary on anything that it's in the Recommended Items tab for in game, and not exclusively either. Pretty much almost any int hero can benefit from it, and it doesn't stop there. Squishy hero players would do well to consider it every time, since single target cc's are very common, and become priority picks the further you advance in rating.