The problem is not only the ability of the Line Training Captain. He is just the last link in the system.
Do you think line training flights are as safe as flights operated by a normal fully qualified crew? I would say no. It is that simple. A lot of people would not voluntarily board a line training flight.
So what do you do? You put in a system where you have a safety pilot and he or she is there for a reason. It is their responsibility to increase the alertness within the crew, since the line training captain cannot catch every little thing that goes wrong.
Lets assume the trainee can be forgiven for not monitoring the FMA and basic parameters like airspeed, should he or she even be given line training in that case? Probably not.
Then lets assume the line training captain gets too busy with other things going on to monitor those vital parameters, which ideally should not happen. What did the safety pilot do? Why was nothing said about airspeed? safety pilot has really nothing else to do but monitor the other two. There is so much more than simply the line training captain.