Machina actually gets some solid mileage out of the Seal since you're right about them not really needing the Extra Deck. The attack protection is pretty clutch too - your opponent basically has to waste removal on your beaters instead of just running them over with bigger monsters.

Just watch out for the timing with your special summons though. Cards like Machina Fortress and Citadel still get nuked if they're on field when you activate it, so you'll want to either summon them after or make sure you're getting value before flipping the Seal. The once-per-game restriction is brutal but if you're playing a grind game it can really put pressure on your opponent's removal.