These function blocks (smoker and blast furnace) are actually well designed: each takes more to craft than the base furnace, but – in return – provides an advantage (faster throughput) without griefing the base furnace.
"smelting item with double speed is nothing special , since its easier to make x10 more furnaces that will do better job" is true in the sense that a number of furnaces or half that number of furnace-plus blocks will produce the same effect for the same marginal cost, but his gives players the option to choose whether to achieve that end via a smaller number of more expensive, special purpose stations or a larger number of cheaper, general purpose ones.
There is also the countervailing 'cost' that the specialty versios each appear to be limited to a subset of the operations performable with the base model furnace. This effect will likely be most noticable with 'super-smelters'*1 as these will likely continue to employ general purpose furnaces given the rarity of having a continuing need for 'smelting' very large amounts of a particular type of thing.
This suggests a three tier 'smelting' economy with furnaces as the lowest ('small' amounts of anything), furnace 'super-smelters' at the top (very large amounts of anything), and specialty furnace block (eg smoker/blast furnace) arrangements in the middle (modest quantities of commonly held 'special' items).
(In that 'super-smelters' tend to be laggy (many hoppers, minecarts, redstone clocks, etc) the introduction of these blocks may reduce the frequency and size of such builds with a concominent decrease in lag.)
*1 'super-smelters' is used to mean arrays of smelting blocks where raw materials and fuel are automatically distributed and finished product automatically collected as distinct from the simpler 'wall-of-(manual)-furnaces'
[As many of MS/Mjs 'improvements' for 1.14 are thoroughly excreable, this one I think they got right.]
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.