Who told you that you don't have to manually recharge your new battery.
Give them and uppercut !!!!!!
The rectifiers fitted to outboards ( very large 4 strokes excepted ) do NOT fully charge a battery. They offer minimal charge to keep the battery topped up.
I have a 15plate N70ZZ ( truck ) battery. If I use the boat on 2 to 3 night trips, I would definately manually charge the battery before the next trip. This charging takes place over a 2 or three day period as I only use a 4 amp charger.
I know people will say that using a sounder, bilge, lights etc, will not flatten your battery, but, the rectifier will not replace the charge you have used, unless of course you are travelling 100 klms back to the ramp.
Certainly, a isolator switch cannot hurt to switch off the power at the battery when the boat is not being used. Please, do not forget that a spike protector is probably more important for your electronics and about the same price as a isolator switch.