Even though I think the summon/sell/buy/summon mechanic wasn't intended, I don't think it gives you that much of an advantage.
I play a pretty decent spirit-Oak and I occasionally do that myself. Usually when I have enough gold to upgrade my idols. When lucky they last a couple of engagements. Usually some AOE skills take care of the weaker minions, though.
Now, when I have all level 4 idols I could go back to base, summon level 4 minions, sell all idols, buy level 3 idols, and summon all level 3 minions. But apart from the gold I have to waste for that trick I also have to wait 30 seconds between summons which is more time than I'm willing to sit around doing nothing. And when I summon them in the field, away from the health crystal, I have to pay the substantial mana costs.
All in all, doing that gives you a minor advantage in exchange for gold, time, mana and extra micromanagement.
Might even be worth the expense, but most of the time I find it too bothersome.
YOU on the other hand don't see the drawbacks because you don't play that way. You only notice the times where this tactic works. And like everyone here that shouts IMBA, OP or whatever, you probably didn'tbother to test your observations in a game to see for yourself how powerful this strategy is.
I hope that unlike Elvisman2001 you don't behave like a spoiled brat and refuse multiplayer just because you can't cope against strategies that seem unfair.
Final words: When fighting against Generals with strong emphasis on minions, buy the Parasite Egg. With that item one Spit and a few blows should be enough to wipe out the general's entire army.