The LM actually landing on the Moon was one of the biggest single and potentially unrecoverable risks in the Apollo program; all it would take would be a boulder or unobserved pit to cause the lander to tilt or tumble, and as was discovered on Apollo 11, the prox system had difficulty in providing good feedback. Doing the descent, separation, and ascent proved out every part of the LM including the Lunar Module Ascent Engine (LMAE), which was ‘sealed’ at the factory and never functioned until ascent from the Moon, allowed the program to prove out every part of the system prior to the the historic landing. Of course, had the LMAE failed then Cernan and Stafford would have been stranded or crashed (and they actually had a control problem with the spacecraft just prior to separation of the ascent stage), but I’m sure that the perception is that it would be better to have such a failure during a ‘test flight’ than with the crew that was doing the first Moon landing. There are actually a number of serious failures and anomalies which put the crew at risk that occurred during the Apollo program but Apollo 13 gets the attention because it was such a public failure and resulted in the loss of the main mission objective.
Also, it should be noted that the LM development program was a slow motion disaster; the frequent design changes (primarily to minimize weight) make configuration control a nightmare, and the first units that Grumman shipped to NASA were incomplete, often with missing or nonfunctional wiring harnesses. The LM was doing something that no spacecraft, crewed or uncrewed, had ever done before (land on another body and then return to space) and there were so many uncertainties and shifting requirements that no two LMs are actually the same configuration, although LM-10 through -12 (for Apollo missions 15 through 17) are pretty close. Trying to rush a fully functional LM for Apollo 10 would have incurred more risk, and delaying the flight would have little advantage versus using it as a ‘dress rehearsal’ for an actual landing. Apollo 10 actually had a number of issues come up during the flight as well has vetting out procedures for the Apollo 11 landing, and given how compressed the Apollo “full up” testing schedule was getting any practical exercise of procedures and hardware was always beneficial.
Space enthusiasts often inveigh NASA and their “overly cautious” safety culture but in fact so many things can go wrong which will result in unrecoverable loss of crew and vehicle that it takes an extreme amount of engineering and oversight to have any expectation of a successful mission, and in the space missions in which NASA has lost astronauts (Challenger, Columbia, and while it never made it off the pad, Apollo 1) the causes have invariably been oversights that could and should have been corrected but instead slipped through because of institutional blindness to clearly defined hazards. The Ron Howard film about the Apollo 13 accident gives the inaccurate impression of improvisation saving the day but in fact the contingencies for loss of the Service Module had been studied extensively and procedures drafted for just such a failure.
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