i thought about this too, but looking closer this would probably hinder your game. except on abundantly habitable planets you can't wait with citizens in colonizers to find the best planets.
example: you check the class 11 planet find no bonuses, move to the next planet still no bonuses, move to the class 6 and find one small research bonus. so colonize it while the AI gets the class 11.
this scanning device would also take valuable room on a ship. the colonizing phase is relatively short, after that a planet can only be taken through war. with the competion for planets i don't think we have the luxury to pick and choose. you inevitably will pass on a planet, and loose it; result is less population, less income. i am in a game right now on challenging. i have only a few bonus tiles, but i am far superior to any of my opponents because of better colony management. if i had been scanning for better planets i would have passed on everything and surely the AI would have the advantage. great idea in theory, but i doubt it would prove much benefit in the game.
population is the number one variable for success. settle the best planet you can as quickly as you can, and start supporting your new colony. everything else is just a lovely unexpected bonus. i could never afford to pass on anything over class 7. with each step below abundantly habitable planets, colonizing ANY planet becomes more vital!
i admit this feature could be effective if given to your flagship. deciding the direction to go between similar planets would be helpful, but not as a way to leisurely skip around the galaxy looking for the best planets. if used wisely, this could be a great benefit, but i wouldn't want to become too dependant on it.