Possible reasons for the recommendation to switch of the alternator field for ground power starts.
1 . Voltage transients created when switching between GPU and battery potentially damaging diodes in alternator (think jumper leads and computers in cars).
2. GPU not appreciating being 'charged' by aircraft alternator.
Piper does not recommend the
alternators be switched off.
It is the
master switch and any avionics switches that needs to be switched off during connection and disconnection of the ground power supply to prevent sparking and damage to the aircraft electrical system.
If a GPU battery start is attempted with the master switch in the on position the battery may be connected in parallel and drag the power of the GPU down to its level. That is if you try to start with external power because of a low aircraft battery and leave the master switch on you may experience very low crank rate due the system being dragged down to the lower level of the depleted battery. In cold conditions they state you can use the GPU battery in parallel with a charged aircraft battery to prolong cranking time.
The Piper variants I'm familiar with have checklists that require confirmation of the master switch in the on position before start. Piper refers to the combination split Bat/Alternator swith as the Master switch in most cases unless seperate alternator switches are fitted. Master switch on refers to both battery and alternator on in the split switch situation. After start the checklists call for a simple check of alternator output.
The alternator off switch is provided to switch it off if it fails to prevent drain of battery charge attempting to excite the dead alternator.
As far as load on start the actual cranking of the engine and accessories would far outweigh any load from the alternator itself. Switch on and off the alternator after start to see how much power it drains from the running engine, there will not be much.