Figured someone wasn't logged in when editing that page. You were the IP who was editing that same page then?
Just a note though, the ratio is not 5:2:1. It is in terms of blocks, but that ratio is in terms of mB. Obsidian gives 288mB instead of the standard 144mB from ingots. The number found in the Mighty Smelting book refers to the amount in mB and not the number of blocks/ingots necessary.
If you hover over a liquid in the Tinkers' construct smeltery, it can show the value in mB, nuggets, or ingots. the nuggets and ingots are actually a multiple of mB (nuggets being 16mB, ingots being 144mB). Besides Tinkers' Construct, there are many mods in Feed The Beast that use this measurement system and has just become a standard measurement of minecraft especially when it comes to mods and liquids.
Outside of this description, i'm unsure what you don't see a use for.
Well, theres more to the smeltery than just blocks and ingots that can be smelted down. There are items of that material type that can be smelted as well, that don't give a "1 ingot" type of value or "1 block" type of value - not to mention, when it comes to blocks, blocks of iron/etc cost 9 ingots, like anything else, whereas the obsidian gives 2 ingots (288mB) worth instead of taking 9 obsidian ingots.