OK, since we are all getting along so well I want to add some additional comments.

Let's say you set the superheat correctly at the evaporator. That's the evaporator superheat, right?

Now let's say the suction line is very long and the insulation is less than perfect (that never happens right?). Now as the superheated gas flows back to the compressor with the evaporator superheat in the vapor, the vapor continues to absorb additional heat from the ambient.

At the suction service valve we can measure the superheat at this point. This is the suction superheat. The higher this gas temperature becomes also decreases the gas density and lowers the mass flow pumped by the compressor. Therefore the compressor capacity decreases. Compressors just pump gas volume. They don't remove kW or BTU's.

Then the vapor enters the motor on a semi-hermetic compressor. This also adds heat to the refrigerant vapor.

Consequently, the higher suction temperatures increase the discharge temperature of the vapor.

Now for the big question....

Why don't we set the superheat on the evaporator to a slightly lower setting to reduce the suction line superheat to improve the compressor performance and reduce the motor and discharge temperatures?

Lower motor and gas temperatures would increase the life of the compressor and improve the oil stability. The compressor runs cooler, so it will last longer.

A slightly lower evaporator superheat means you are using more of the evaporator surface for latent heat transfer, so you are picking up more heat with the same coil surface.