Wrong.
The following capture is directly from Narutopedia. Yes, not that it is 100% accurate but so far, it is one of the most reliable sources of information.
"Hidan's main advantage was his inability to die, a result of extensive experimenting with the Jashin religion's various techniques. His immortality allowed him to survive numerous fatal injuries, and almost any form of dismemberment; he even retained his ability to speak after having his head cut off. However, his head still needed to be connected to his body in order to control it.
Despite being "immortal", he is still capable of dying from a lack of nutrients."
It says nothing about him needing to perform the ritual in order to keep being "immortal".
And to someone being unsure how immortal he really is, it is stated that he can still die from lack of nutrients. Which are water and food. So by now, he is pretty much dead in that pit that Shikamaru buried him in.
He is more than worthy to be a part of Akatsuki. It's not always about "OMG NO ONE CAN SURVIVE AGAINST IITTTACHI <3 ALONE LOLOLOL!" or something similar. It's about being the most effective as a unit. They did capture a Bijuu together, did they not? And since Kakuzu had the habit of killing his partners, Hidan was the perfect choice to company him. They used Hidan in Kakuzus elemental attacks, that would've been impossible for someone else. So Hidan is nothing near useless, quite the contrary.