Telnet duplicates the functionality of CTP (41795 aka what Toolbox uses by default)

It’s handy for connecting quickly, especially without using Crestron software (IT people pretty much universally understand “Telnet to…” but get a bit shakier when you’re specifying “nonstandard” ports for Telnet to connect to)

I would not expose processors (port 23 or otherwise)  to untrusted/potentially malicious traffic (e.g. WAN or public/student VLANs in academic settings) and would tend to recommend securing the network upstream of the processor.

BUT… On the 2-series if you configure a password (text console command, surprisingly, “password”) you should be prompted for a password when you connect via 41795 or 23 – but there is no logging, no lockout, etc. so it is vulnerable to brute force attempts (again… secure the network first)

The 3-series has more advanced authentication/authorization/accounting capabilities, but I haven’t spent enough time playing with them to feel comfortable making specific recommendations.

Useful links: 

Security Reference Guide: Crestron Touch Screens

Ethernet Ports (crestron.com)