How big is their main motor?
Sometimes some of the bigger or older outboards don't like to idle down slow enough for a good troll...to fast.
Guess it would depend on what type fishing you do.
If your main outboard were to break down,a 2hp would move you faster then almost any affordable electric.You would not have to worry about batteries just gas.
Trolling motors are used for mostly bass fishing styles.If I were to be trolling salmon/trout/stripers etc.,I'd look into a smaller outboard,if my main would go slow enough.