I always loved a full fire TB. I think the inverse of the OP though, I only put one point in Ring of Fire at the beginning (so that I can zone flags, take out a crowd while Heaven's Wrath is recharging, or work the first lane tower in Cataract in early levels), and then max out Fireball and Fire Nova. The reason is that it's a lot quicker to clear crowds of creeps with a Fire Nova than it is to hold a crowd until Ring of Fire eats through them.
From there, though, it depends on the situation as to whether I choose to place points into RoF or Enhanced Attributes. If there's a good Reg or UB on the other team, I won't waste my time on RoF since Reg will run right through them or shoot me out of range, and it won't even phase a well built UB. If I'm left to my own devices though, I'll invest in it in order to take out multiple towers or Rook farms faster, or to help out my mates if they're getting harassed (early game at least, since a fireball+burst makes a quicker impression).
To me, though, it's just far too situational to rely on. It's safer, arguably faster, and more mana friendly to fireball towers down out of range when I'm pushing towards a portal in cataract alone (not to mention a lot less detectable), and even better to have a faster attack speed when I have people helping me push. If I'm saving people or soloing a UB mid to late game, I don't have time for big casts when everyone has stun and/or people need my help quickly: I need high burst damage and a fast rate of attack. On top of that, Ring of Fire is easily detectable and can either push potential kills away if I have the advantage, and basically is offering a free kill if I have the disadvantage. Most of the time, it's asking for a death sentence rather than security.
On retrospect, it definitely seems to be the worst (albeit not "bad") skill Fire TB has to offer, and I rarely have reason to use it late game unless giants are at our doorstep. Whenever I do, the first or second level of RoF suffices since they're usually used in games of cat and mouse and rarely deal the entirety of their damage to an enemy demigod, and just the image of the cast can be enough to scare off weaker players.