Let's put it this way: phasers can hit basically anything that isn't well-cloaked and is in range with 100% accuracy. They also pack a big punch.
This means that, more or less, the only way to get a physical payload to the target warship is to overwhelm it with numbers in too short a time for the phasers to hit them all. Since it is basically guaranteed that a single hit will kill such a small target, this boils down to rate of fire for the phasers.
So any fighter or strike craft will basically get blasted before it can get close enough to do anything, but if there are enough strike craft in the attack, then some of them may survive long enough to attack the target before being destroyed (and some of them may even survive long enough to make it back). At that point, you may as well be firing missiles that devote more of their mass/design towards being better missiles than wasting anything for a pretense of "making it back to the carrier".
A mobile repair station and such is perfectly fine, and makes sense. Because it's logistics, not combat, so you don't have firepower or shielding enter the equation at all.
But otherwise? It makes far more sense to have something like a Defiant-sized warship that maximizes firepower, speed, durability, and ease of construction/low resource cost by being small and doing away with stuff that isn't necessary. If the Federation had been churning out Defiant-class warships like the US was churning out M4 Shermans in WW2, then the Dominion War would have looked quite a bit different.