The necessity for tabs is dependent a lot on how you use the boat. If you boat on a lake, with no chop ever, and have passengers that are perfectly weight balanced at all times, then you probably don't need trim tabs.
The advantages of tabs are: they give you the ability to control the longitudinal attitude (list) of the boat while underway, since they will force the bow up or down and/or to port or stbd based on how they're configured side to side. They also give you a better ability to control the overall up/down angle of the bow, independent of the motor trim, which can lead to better efficiency.
The disadvantage is: you have to install trim tabs. There's an monetary, effort and space cost associated with that. If that doesn't phase you, then tabs are a great investment.
Having the ability to push the bow down into a choppy sea can make the difference between getting bounced around, barely on plane, and fighting back to port while getting beat up, or just being able to bury the bow, slow down, and enjoy the relatively calm ride. You can do that with the motor (a Permatrim helps a lot), but having the tabs gives you another option. If you have the space, money, and ability/more money to have them installed, I'd recommend them.