Fundamentally, every starship in Starfleet is expected to undertake science duties. That's Starfleet doctrine, that's why every ship is an absurdly overbuilt cruiser. (Yes, even the Defiant.) If Starfleet wanted to design a utilitarian escort, we would design a Bird of Prey, which can be operated by a crew of twelve, and trade on advantageous terms with any Federation starship in service. But we don't, because that isn't Starfleet doctrine.
Adding science bays gains us a whole extra rating, when nothing else we have can do the same. Extra transports or extra medical facilities make the ship better at responding to crises, but as a Federation starship, which is useful, it's already extremely well-equipped at doing that at a baseline, with a level of provision that most other races would consider remarkable. Science facilities will make the ship better at doing scans, surveys, and other routine tasks, which is a mission it will be doing every day, alongside its regular escort duties. It's a significant increase in the utility of the ship, especially because the jump from D to other tiers represents a much bigger difference than going from say, B to A tier.
Like, I think there's an idea here that from a utilitarian, "min-maxing" perspective, what we should do is neglect science... but the utilitarian answer is actually absolutely the opposite! Adding labs this gets us the largest statistical improvement to the ship's stats any option. It is a given that these capacities will be used if they are built, because there is not a single Federation starship in service which does not do science as part of its mission profile.
