That's a good point on the limitation, and to expand on your last point, an immediate demographic this could help is frontline healthcare workers and similar jobs who do get routine testing and are at higher risk in general.
I'm a teacher and vaccinated. If I found out that I was exposed, and then tested positive... post-exposure prophylaxis sounds great.
Also there's my dad, who has an immune condition due to old age. He's vaccinated, but who knows how effective the vaccine was for him. The existence of PEP could make it possible for him to do more at a similar level of safety... vs. the current option of staying in a small bubble for the rest of his life despite being otherwise able-bodied and capable.
https://www.covid19treatmentguidelines.nih.gov/therapies/ant...