It does kind of suck you have to hit the rear defrost to activate it though. Lots if times there's ice build up on the front as you're driving into crap weather and the back can be clear and not need heat. My wifes 2021 Rav has a button to activate just the heat up front, wish mine was like that.
As long as you are aware of how to activate it, I don't think it hurts anything to run rear defrost when it's not needed. I regularly run my rear defrost when the windshield and rear window don't need it. Why? Because the defrost heat turns on the side mirror heaters too, and if they are wet (from dew/rain) the heat from the defrost will dry them out as you drive.
A separate button I wouldn't like, cause that's just another button I need to remember to press as needed instead of just one.
Is the RAV4 a heated windshield designed to help melt ice on the visible portion of the window, or only to keep the wipers from freezing? Most vehicles have wipers sitting on the visible portion, up where the defrost vents can blow. But the Sienna uses a heater grid for de-icing at the wipers because the wipers sit down low, below the clear section, where the defrost vents can't reach. Hence why, I believe, all Siennas have a wiper defrost heater. But some new vehicles are starting to add de-ice/de-fog to the
clear portion of the windshield, for faster windshield clearing. This hasn't been a common thing due to cost. Airplane windshields, at least those of commercial aircraft that fly in icing conditions, have whole-windshield clear de-icing electrical heaters. That's why they have a gold tint to them if you look at them. But at least historically, this technology was too expensive to apply to "mere" passenger vehicles. But this clear de-icing tech is very, very similar to the clear touchscreen tech on your cell phone screen, and chances are the surge in touchscreens has made such tech much cheaper and more suitable to integration into things like vehicle windshields, and the de-icing lines on the rear windows will someday be a thing of the past, and windshields will all have whole-window electrical de-icers that are perfectly clear.