There is some Exp for completing a Writ, so you get that.

Yes, the Inspiration you get does you no good if you are max level already in the craft line. But, one thing to keep in mind, you can usually get to the max level of a crafting line long before you max out the crafting line. Like with Enchanting, you can make max level Glyphs at Enchanting of L40. So, there is still room to grow there.

Besides, doing Crafting Writs in order to level Crafting is doing Crafting Writs for the wrong reasons. Because even at low crafting levels the Inspiration you get is barely enough to move the bar. You would get more Inspiration in less time deconning a few looted items or making a few foods/potions. I have one character doing L1 Writs in all the professions. She has gone through a stack of wood, a stack of iron and a stack of Jute (200 stacks I might add) in the past week or 10 days. I think she has gained one level in each profession from the writs in all that time.

Do the Crafting Writs for the rewards, which for the Consumable Writs is quite nice of a return on investment. For Equipment Crafting, not sure why to even do those. You get a small chance to get a Gold Temper (I have yet to get one) and maybe, just maybe, a max level Writ will get you a survey that gives you a .00025% chance of getting Nirn-something from a Survey.

To summarize, do the Consumable Writs for the rewards and to restock your larder. Those are worthwhile. Do the Equipment Writs because your a masochist who enjoys spending time doing something that costs you resources with no return of any consequence to show for it.