Mustrum_Ridcully is really hitting the nail on the head IMO. It's not necessarily having the 15 MAD that's the problem. After all, there are all sorts of scenarios where it's not a problem to have one big fight and then fall back. I'm not saying that this is the only scenario, simply that these scenarios are not all that uncommon either.
The problem is that the Vancian casters and the non-Vancian characters are playing fundamentally different games. The balance of the game can radically change depending on the pacing of the game and this can be an issue in some groups.
There's a reason that some groups adopt, or try to adopt, the 15 MAD - it's an effective strategy. IMO, the best way to reduce 15 MAD is to reduce that effectiveness.
AD&D did it by having very, very strong restrictions on casters and by having martial characters that were far, far more powerful than the opponents that they faced, at least on an individual level. It wasn't unreasonable for a fighter type to get through a combat without expending any resources, or at least minimal enough resources to ignore. So, why bother going nova with the casters when your fighter types can just blow through the encounter anyway?
4e went another direction and made everyone have the same resource base.
I'm not really sure where the proper answer lies. But, I do think that simply giving advice isn't it. I went around the block several times with [MENTION=19675]Dannyalcatraz[/MENTION], with him showing far more patience than he probably should

, trying to show me how he gets the results that he gets. Now, I still don't understand. I have too many questions to be able to adopt his advice into my games.
Granted, even at the times when we did have 15 MAD, I never really saw it as a problem because I just thought that that's how D&D is played. IME, it's always been there as an option and, because it makes a lot of strategic sense, whenever the situation allows, I'd pretty much presume that that's what the party would do. It's no different than sending scouts ahead or searching rooms or any other standard operating proceedure that groups fall into after a fairly short time.
3e makes it a much larger issue since the casters get SO much more powerful at higher levels. Virtually any problem that comes up becomes an exercise in patience and throwing enough magic at it. And I can see how the non-casters can feel very sidelined by this. Watching the change in one campaign where a player playing a paladin who died brought in a cleric as a replacement character that suddenly dominated virtually every encounter was a real eye-opener.
We didn't really have 15 MAD for most of our games, but that's because we didn't play core casters very much. No cleric, we had a Favored Soul. The druid in a later campaign had reserve feats. And the running joke in the group was that wizard was the cursed class because every wizard player wound up quitting the game after only a few sessions.
Honestly, my solution to all this would be to go that direction - use things like Favored Souls or Shadow Casters (from Tome of Magic) as the baseline for casters. Sorcerers as the model for Vancian magic instead of the Wizard. It reins in the power creep from extra books and keeps everyone on a much more even footing for resources.
I'm tired of half my players playing different games.