Cubic Ninja seems to be patch-proof. It's a nice backup to have, it will always run.
Nintendo could just delete the old ironhax files from their servers, so you might not be able to re-download it later should you ever need it again
In this case CN would only be useful for if you ended up accidentally updating your sysnand, or if you did a stupid and deleted your browser.
As long as you'd remain on 9.0-9.2, sure, CN is okay to have I guess. But you have so many other options that work just as well. browserhax will always work on 9.0-9.2, and menuhax offline.
And if you ever accidentally lose menuhax or any other entry point you might be using (oot3dhax), browserhax will always be available to let you re-install them.
It's just not worth spending so much money on CN anymore. Users on 9.9 or above who don't have an entry point can go ahead and update to 10.3 and secure access to Ironhax and Menuhax until browserhax is made unavailable, and then downgrade to 9.2 where they can do everything and Cubic Ninja becomes highly redundant. On 9.3-9.8 you can just carry on using browserhax as normal.
It'd only be really useful again if you somehow got stuck past 10.3, and even then, might as well sell that 3DS because a 3DS on 10.3 or lower would be stupidly easy to find.
Honestly if you already have CN, then that's alright to have. But if you don't, it's definitely not worth spending a cent on it to buy it at this point if you ask me.
How good is having CN when you have browserhax which does the same thing, for free?
Ok, the one use I could see for CN was if you did a stupid and deleted your browser from sysnand and didn't have menuhax set up nor any other entry point.
Then you'd be able to use CN to get into your CFW again and re-install the browser.
But no-one's ever gonna be in that situation, not when menuhax is too convenient for someone to just use browserhax for everything. If you deleted your browser you could just get back into CFW on sysnand and re-install a .cia of the browser as long as you didn't do a double stupid and remove menuhax after uninstalling the browser.