As you've already noted, when you have suitable transformer secondary protection, you are _permitted_ to have primary protection up to 250% of the transformer rating.
The reason code permits this is to prevent OCPD tripping on the initial inrush when the transformer is energized. This inrush current can easily be 10-20x the rated full load current. This inrush is also very difficult to predict, and depends upon things such as the timing of the AC cycle when the transformer gets energized, the impedance of the transformer, the impedance of the supply circuit to the transformer, etc.
I don't think that transformer OCPD oversizing has much to do with downstream connected motors, but I suppose it might apply if the motor always connected and started simultaneously with energizing the transformer.
Large motors on a supply bus are known to add to available short circuit current, and so motors on the _primary_ side of the transformer could increase inrush current by reducing supply impedance.
There is no requirement to oversize the transformer primary, simply _permission_. The closer the OCPD to the actual continuous current levels, the more protection against faults, but the greater the chance of tripping when the transformer is energized.
Keep in mind that the primary OCPD protects both the transformer _and_ the primary circuit conductors, but 'tap rules' come into play. Larger OCPD may force you to increase conductor size.
My experience with smaller transformers (30-50 kVA) is that you can generally _undersize_ the OCPD without problem; say if in an emergency you need to use an available (but oversized) transformer on an existing circuit.
-Jon
