It is quite useful for gathering , as the fruits are bottlenecks for many recipes. That's why I would keep them if you do any lifeskilling at all. 
Additionally, there may or may not be events sometimes where keys are required so I would definitely always keep some of them around (we had it before so now I'm hoarding xD)

The chests in the open world are not really worth using the keys on though. 
Also note that if you want to get your own T10 at any point, you will also need a lot of fruit of enchantment, which you may (depending on the market situation at the time) need to gather yourself. The chests would help a lot there. 

On average you make a lot more by unlocking the treasure chests than you would selling the keys on the market. As Minarya has mentioned, the rarity of the fruits should also be considered because they are an absolute pain to get otherwise. There are some quests that require them as well and when you suddenly need 100 of a rare variety it can make you feel impatient pretty quick! I recommend keeping them and using them whenever you can. Even if you don't make quite as high as you would have selling the keys, the peace of mind would have been more than worth it, but on average you do make more than the value of the keys.