Here's what I've found is useful so far:

1.  Send minions down a path while you go capture flags.  Sedna does this well since she is fast.

2.  Depending on which general you are, send minions one way while you go the other way and attack.  QoT does this well since she has that aoe.

3.  Leave minions to defend the tower area while you go to the shop.  Depends what the armies in the game are set to, but more often than not your towers won't take any damage with the minions there to support them.

That said, minions alone aren't going to win you the game directly.  I use them for various kinds of support like the above and that's pretty much it!

I am going to have to disagree with the other 2 posters. If you do what you said, and focus on your minions, I have found them to be very powerful. I can absolutely rape towers with the army and when you suprise a demigod (teleport into a battle with your army) the very large change in DPS often nets you a kill. I think the problem is that people are playing too many hybrid builds and thinking its a minion build. This leads to people thinking minions are underpowered. But let me tell you, with 12 melee minions, the right EQ and Erebus in mist form, you can get a HELL of a lot of DPS going.  You just need to focus on your minions and take equipment that compliments them well. Just taking something because it has +minion X on it wont work, just like picking items for your char willy nilly wont work. With the right items I can get my minions doing nearly 100 damage a hit each. x12 and you would be suprised how well they act as your primary DPS.

Are they better at killing demigods? Of course not. That would be rediculous. However, they dilute targets, they deal great damage, they molest towers and the citadel and if your teaming up with an assassin style demigod, you can advance without waiting for your creep wave (this last one is important as it can let you advance where as otherwise you would not be able to).

I use Oak with full spirits, soul power, and minion boost skills. Combine that with all level 4s and you have a nice army (as long as you put down the flags in not stupid spots). Spirits can be sent on suicide missions and still do nice damage if you can keep them coming, also remember to plant flags if its only your side against some structures, you can still get spirits from dead good guys.

They're really good for not purchasing. That way, you have a lot more gold and skill points to spend on more useful things.