My own experiences....
Form is destroyed (risk of injury)
Concentration on the lift is lost as you are concentrating on that last few reps (risk of injury)
Longer to recover (risk of injury/overuse)
Typically exercises where you go to failure often start to become ballistic (attempt to move the weight quickly even though not possible) even when consciously attempting to go slow (more ballistic, more risk of injury)
Taxes the nervous system, maybe excessively (risk of injury)
Training to failure enables the feeling the burn sensation. Once experienced the sensation, a training session starts to feel inadequate unless that sensation can be replicated.
I have experienced all of the above.
Russell, I have been in the rut you speak of. It's a sad and lonely place.
I personally train to maintain (and build) strength as my age increases (ok, i'm 31 but eventually it will catch up with me) and remain mobile through ROM as long as possible. Supporting your own bodyweight will soon become a challenge for everyone, shame people don't realise it more often.
I know I repeated a lot of previous posts, it's always good to emphasise
Follow me on twitter (if ya interested) @HalfofTDB