Better to get the field disconnecot. One mistake, one disaster
averted, will illustrate the advantages.

Any make before break a / b switch will enable you to switch batteries
around safely during engine run time, but only the field switch will
protect your alternator and electrical system from switching battery
load to off during engine run time. Move the switch more slowly than
rapidly.

For 10 bucks its very good insurance and a valuable utility.

Terry K