They give you crafting materials that help you level up, they are worth your time to complete if you want to be a crafter.
Plus you can sometimes get gold tempers that sell for a bit of gold too.

Sure, for the drops alone. I get some good drops on my Enchanting writs (2 kutas over the last week). Every very other Provisioning writ or so I get Frost Mirriam and Bervez Juice. I have never seen these two items anywhere besides my writs.

Blacksmithing, Clothing and Woodworking get so so drops. I just started Alchemy, but I like the idea of being able to craft my own tri-pots eventually.

If you plan on crafting your own gear, potions or glyphs the writs are great ways to increase your levels relatively fast.

Just my 2 cents...

Just doing the writs alone won't raise your crafting skill lines significantly enough. You still have to decon items and craft potions/food/drink in order to level these skills. However, in the early levels they are quite useful to help raise those skills and provide some good materials.

As others have said, the equipment writ rewards can be hit or miss, since sometimes, after using up a good number of mats, all you get is a trait stone and a white piece of gear. Unless you're lucky enough to get a survey and/or gold temper, the gold reward is basically it (besides the inspiration gain, if you're still leveling). Granted, if you don't have trait stones coming out your ears, then that reward is ok, especially for a starting player. And, if the white gear is intricate, that's great for someone still leveling their craft. Ornate is better if you're no longer doing the writs for leveling, since at higher levels it's a decent addition of gold. You can also get repair kits, which may/may not be useful to you.

Provisioning is by far the best in terms of cost/reward. You only need to craft one food and one drink for the writ, and can make stacks of them beforehand, making them the easiest to complete - provided, that is, you have the recipes needed to complete the writs. What you get in return isn't always spectacular, but it's a great source of basic mats if you're still working on building up your stock, and you can get new recipes. At worst, you get 3/4 basic ingredients (which I usually toss since I have full stacks of all of those) and a green recipe that you already know. But, you can get blue/purple recipes, which if you already know, you can sell more easily and for more money than greens. And, if you get purple ingredients, you can add to your stock (or if full, like me, you can sell them). The best is if you also get an Ambrosia Fragment, but the drop rate for those is abysmal - if you do get one that you already know, you can still sell them for 25-35k gold.

Alchemy is next in terms of cost/reward, as it calls for only one potion (which you can make stacks of ahead of time) and 3 of one kind of ingredient. If you don't have the requested ingredient, you have to go harvest them, so starting players will have to do some more legwork to get the required ingredients. The reward is always three of the current level of solvent and 3 each of two different ingredients. And the drop rate on surveys is much higher for alchemy writs than it is for the others. On average, I get surveys about 50% of the time. So, you get a good chance at surveys to lead you to additional ingredients.

Enchanting is the most difficult of the three consumable writs, since it requires one glyph, which has to be made by you and since they don't stack, you don't really want to make these ahead of time. It also requires a specific rune, which can be more difficult to find than plants, since all enchanting nodes have a chance of spawning one of the three types. So, you have to be lucky to find the right type of rune you need (Aspect, Essence, or Potency) and then you have to be lucky that it will drop the specific one you need (Ta, Deni, Jode, etc.). And, the rewards aren't as good as the other consumables. You do always get an empty soul gem (1-3) of the appropriate level as well as a green glyph that you can either use or decon. However, if you're lucky, you can also get a Kuta as well as a survey. Unfortunately, in my experience, the surveys don't drop all that often. But, I have been lucky to get a Kuta in the writ bag as well as 2 Kutas from the survey (never been lucky enough to get more than one Kuta node, and never a bonus from the CP passive - well, one time I did get 2 Kuta and 2 Denata from a survey node).

TL:DR - writs are a good supplement for leveling crafts and really good early on as a source for materials. Later on, they can be an easy source for gold and a good source for rare mats.