Mostly question in title.
Currently, the Divine Smiting enhancements improve the following spells:
Nimbus of Light
Searing Light
Divine Punishment (when U9 goes up)
Deific Vengeance
Holy Smite
Chaos Hammer
Order's Wrath
Unholy Blight
What do all of those spells have in common? Two things.
1) None of them (except the one that isn't out yet) are above level 4. The one that isn't out yet is level 5.
2) Most of them (except the one that isn't out yet, and Searing Light) are -bad-. They either just do woeful damage in general, or have significant restrictions on what targets they can be used on...or both.
This makes people disinclined to bother investing in the Smiting line at all...there just isn't a lot of payoff for doing so. Primary spell offense comes from things that the Smiting line doesn't do anything to improve: Blade Barrier, Cometfall, Destruction, Implosion.
But now we have Update 9, and the Angel of Vengeance PrE, as well as the spell pass. The spell pass drastically reduced the SP cost of the following spells:
Symbol Of Flame (cost reduced from 25 to 10)
Flamestrike (cost reduced from 30 to 20)
Firestorm (cost reduced from 40 to 25)
As it happens, that's every fire based spell in the divine arsenal. Turbine is clearly recognizing that these spells are almost never used at all, and is trying to make them more appealing.
The Angel of Vengeance PrE includes built-in boosts to fire spells at the second tier. Presumably, when the Cleric offensive casting PrE comes out, it will do something similar. So here again we also have Turbine trying to get divines to use fire spells. Ok then.
There are 3 reasons why those fire spells don't get used very often:
1) No enhancement line to boost them
2) Without an enhancement boost, there's not much reason to carry around a Combustion/Fire Lore item, since the damage is already bad, it's better to just not use the spell at all.
3) From level 15 on, late and endgame was heavily dominated by devils who are fire immune.
With only 3 fire spells, that's not enough to justify an entire enhancement line, and as previously mentioned, the existing spells on the Smiting line are...pretty bad. Folding them into the Smiting line will give them a much needed boost to make them viable killing tools.
With an available enhancement line, clerics and FvS who look into these spells might well invest in fire boosting equipment for them. More gear options! More grind! More inventory space taken up! Woohooo!
As more packs are released, and with more epics becoming available that do not revolve around devils, the list of targets where fire actually works is becoming larger.
I'd certainly not advocate throwing the ENTIRE offensive spell list into Smiting. If there was going to be something to boost untyped/physical spells (and there's more of those than you think), that -would- justify its own enhancement line, if only for Blade Barrier.