These servers are running Linux. Linux is NOT perfect, and an occasional reboot could not hurt anything. I would of course make a FULL backup before a powerdown/reboot, in case it doesn't come back up, and you need to get it replaced.
The danger in not rebooting these S-Series servers is, you will eventually have a powerfailure, if your Linux boot record is corrupted, or any other # of problems occur, your system may not reboot. If you plan the reboot, and make a full backup on an external device/machine, via FTP or SCP or SFTP, then you will be prepared, in case it doesn't come back, rather than be in a panic after a power failure or catastrophic failure of the S-Series server to come back up.
AVAYA has recomendations for the IALX to be rebooted every 180 days, it used to be every 90 days (was easy to say do it with every new season). Of course they have a "242" day bug, if your audix has been running for longer than 242 days, it will no longer answer calls and must be rebooted. This was their solution to the issue
I should say I work for a Business Partner, and we usually end up powercycling a machine at least twice a year, due to scheduled building power downs, for the most part.
Mitch
AVAYA Certified Specialist