Having heroes filling in the gaps in your party skills makes it easier to have a highly capable party while still being an effective combattant yourself. I disagree that there should be more party slots though, or slots specifically for heroes. Since having a hero in your party boosts the entire party (e.g., by making it faster, giving it a wider perception range on the map, etc.) they can be more valuable than regular units, so there should be a trade-off. I think that decreasing by one per hero the number of varieties of units you can have is a fair trade.
Of course, this means that if more heroes are added to the game, you don't necessarily want to have all of them in your party, and I think that's fine too. There are only so many party skills a hero can offer, and since most of those skills don't stack, there's no point in having more than one hero specialize in them. What I imagine is having new heroes become available as the story of the game progresses, and letting the player decide upon meeting one whether to add him in place of one of his current heroes, sacrifice a party slot to add him without dropping a current hero, or simply not adding him at that time.
As a side note, this has already been suggested, but with more heroes it becomes even more useful: it would be great to be able to take one of your recruited heroes, put him in charge of his own war band, and set him loose on the map as an independent unit, either doing whatever he wants or obeying some simple orders you give him before setting him loose. That way unneeded heroes can do something useful instead of just wasting away at the inn.