It is not a waste of time to level up generals, even the ones you get right at the beginning. (When generals hit level 21, they trade in their horse for a tank and get a ranged attack if they don't already have one.) Generals, especially higher-level ones, are beasts on defense. On offense I use mine mostly for clean-up, but if you really need to take down a base, whether for war or just because it's especially rich, sometimes it's worth the cooldown time to use them to lead the attack. Having more generals levelled up helps make it less painful if you can't use some of them for 24 hours.
You didn't ask this in so many words, but it's not worth hanging around in Gunpowder just to max out your generals. The most valuable resource in the game is worker time. You can, at least in theory, always raid more to get more food, gold, or oil, but the once you've built as many houses as you can the only way to get more worker time is to use crowns. If you've still got forests to clear or other things for your workers to do while you level up your generals that's one thing, but it's definitely not worth idling your workers and holding off on advancing to EA until you have absolutely maxed every possible thing.
