Viability of single job crafting

[Seraphor]

You mention Ingenuity I.
What class is that and from what level?
Any other level 15 skills that are worthwhile at endgame?
I could probably stretch to leveling some others to 15 or so for their first cross role skills, but I'm not sure I'd go much further than that.

[Caimie_Tsukino]

You mention Ingenuity I.
What class is that and from what level?
Any other level 15 skills that are worthwhile at endgame?
I could probably stretch to leveling some others to 15 or so for their first cross role skills, but I'm not sure I'd go much further than that.

Ingenuity I is from Blacksmith (BSM) level 15.

CUL is certainly a class that can stand alone just fine. However...

(1) You should at least get "All-15" status, as it takes minimal effort of about 1 day to do them all. But the benefit is gigantic... Skills like Waste Not (LTW 15) and Trick of the Trade (ALC 15) are wonderful to have, and will contribute significantly in assisting your CUL leveling from lvl 1 to 65 (At lvl 65, you will have Manip2, which can replace Waste Not). And as Liam mentioned above, Ingenuity (BSM 15) will also be extremely powerful when you try to take on difficult crafts. Other lv 15 skills like Manipulation, Rumination, Rapid Syn etc are all useful too. To achieve all-15, you just need to buy very cheap materials from Guild Suppliers and/or City Traders and/or FC house material supplier NPCs, and then easily go through some crafting log.

(2) Keep in mind that CUL is probably the one class that uses the largest variety of mats. So it will still fill up a lot of inventory space. But I do agree with you that this is a lot better than taking up all 8 classes if you're concerned about inventory.