It depends on why you're only applying to one program.
Personally I would be concerned that the applicant isn't serious about getting a PhD if they are putting the trajectory of their career on one school, especially since admissions is not strictly merit-based. I would also wonder why they can't find a good fit in more than one institution. Are their interests too narrowly defined? Are they not flexible/open to different advisors or specializations? If so, what if their interests change during grad school, or the advisor becomes unavailable?
Two good reasons I can think of are: (1)
