CSM Options: Icon of Flame

The Tzeentch marked boys have the option of getting the Icon of Flame: an interesting tactical icon that grants the soul blaze special rule to anyone armed with bolt-spitting weapons. 

Now, soul blaze in itself is an interesting idea.  If a unit takes an unsaved wound, there's a 50% chance it'll take a further d3 S4 hits at the end of each turn.  That's basically a mean of 2 extra bolter rounds.  Sure, they still have to wound.  So that means 1 wound (if against a space marine).  And said space marine stands a 1 in 3 chance of perishing from this.  This isn't great, but can be enough to finish off a pesky unit that has been whittled down to the last man (or two) on occasion.  Its probably better against beasts (e.g. chaos spawn), the odd low toughness character (or even monstrous creature), and hordes of tyranid gaunts (in which case, take an icon on every squad so that you can soul blaze every opposing unit!).  It could be worthwhile with havocs armed with bolters, but here's the but:

Given the average price of this icon is about 15 points, we must weigh it up against something like purchasing a heavy bolter (5 points cheaper!) or a plasma gun (same price).  Due to the high S and low AP, the plasma gun is going to win every single time. 

Hence, I might consider the odd Icon of Flame on a (fluffy) Tzeentch-themed army, particularly on havocs or terminators, but that's about it.  Otherwise, in a generalist list, I can't particularly see the worth in them.  Invest in a plasma gun instead.  Or a heavy bolter plus flamer.  Or meltagun plus flamer.  Or a missile launcher.  Any of these are probably going to earn their points back quicker than an icon of flame, sadly.