@Gabriel For me, if there were only the state of perfected one, I would lose options to practise at my own pace. If we go by the early discourses, it’s impossible for any person who wants to become a perfected one to practise as a layperson since complete seclusion from sensual desire is essential, among other things. That would mean all Buddhists must ordain in order to properly practise the teachings of the Buddha. That would be chaotic since there isn’t any lay person who could give them food, and that would mean monastics must break some monastic rules in order to survive (there might be some non-Buddhists who are willing to support the monastics, but I think that they would be very few in number). So, you either ordain in order to become a perfected one, or stay as a worldling and wander in the cycle of birth and death without end, and with high chances of being reborn in the lower realms.
As for your original question, the practical value of other states of noblehood, for me, is options (for awakening). Those who practise Buddhism ultimately want to make an end to all suffering/realise awakening, but some people still enjoy sensual pleasures, so they may aspire to become either stream-enterers or once-returners since these states are available to them. For some, they still enjoy the bliss of absorption, so they may aspire to become non-returners. Though, I agree with you the fact that there’s no way of verifying the first two stages of noblehood is a little problematic (non-returners are without sensual desire and anger, so perhaps they can be verified more easily?). Still, I wouldn’t worry about it too much since it is clear that some things from the early discourses must be gone by (wise) faith. Personally, instead of worrying if I realise this stage or that stage, I am more concerned about unwholesome things that I must let go.
I believe that the category of noblehood is meaningful, and I don’t think it is a hindrance in anyway. Perhaps, for others, the category serves little to no purpose. But for me, first and foremost, it is an explanation of one of the laws of nature. In this case, the natural progression of one who correctly practises the teachings. I think of it this way: in order for a baby to become an adult, the baby must first become a toddler, then become a kid, next become a teen, and finally become an adult. This is the nature of humans. For me, the way that the category of noblehood is presented in the early discourses is the same: that it is the nature of noblehood (the category is presented in a lot of Chinese Agama discourses along with their Gandhari, Sanskrit, and Pali parallels, so I doubt that it’s inauthentic).
On the last point that I would like to make, the Buddha is clear about teaching things that are relevant to the ending of suffering, so I doubt that he would teach about noblehood just because.