Tbh, the only reason the engi restriction exists is for consistency but I agree in that I don't think it affects the gameplay that much. That's in contrast to pyro/heavy/sniper where those classes have way higher skill ceilings so I think a class restriction is way more valid (we've had a lot of issues in the past in lower divs with HL players on those specific classes). Though I can see an argument for why people should be allowed to play their HL classes, it can massively hinder the development of lower level players as they never actually get to play 6s meta. It makes the game really difficult to learn/play if people spawn up HL classes instead of giving the opportunity for teams to learn the meta. To clarify, because we want lower divisions to be healthy in terms of competition/growth, and because HL gimmicks (essentially) do not work at higher levels, it's in our best interest to add restricts as to not turn players away. For example, an advanced team knows how to deal with a pyro/sniper much better than a newcomer/amatuer team (lead with and chariot scouts in fights/bomb and pressure the sniper with your projectiles).
Tl;dr the mechanics and gamesense of lower level players is not nearly adept enough to deal with offclasses, restricting allows for these players to get a sense of the actual game as they attempt to transition to higher divs.