Joel I am willing to discuss it, I also (from your post) am not convinced you arent new to plc's (being new to this site is another thing).
Anyway to start the discussion/debate etc ...PLC's were designed to REPLACE relay logic...why? cost for one, a plc could replace many relays/timers/counters...ONE device to replace MANY,,,,awesome concept. They take up less space and can do the job of MANY devices.
Reliability? compared to what? PLC's are solid state devices,,no electromechanical devices can compete in this aspect.
Relays have a life expectancy dependent on how many times they are opened and closed, plcs do not have that. The timers, counters etc ALL software and cannot mechanically fail.
There are situations where an I/O with a plc can fail but the relationship to a relay etc is minimal.
If you ever had to troubleshoot a double cabinet ( each 6x8 ) that was all relay logic and analog devices then you would understand the difference.
As a project..if possible...use a small plc (10 I/O or so) and create the relay logic system to replace it. I am not as inept as the guys here when it comes to programming devices ( I have to bulldog my way) BUT I do understand WHY they are more efficient.
To make it simple a PLC can replace a LARGE cabinet of relays and at same timne be more reliablw/efficient.