I see that you are apparantly not following me, but I don't know how to express it otherwise.
Maybe my example must be more specific and explicit?
Let's say the party is looking for a murderer. They are following his tracks, interviewing witnesses, and all the stuff. After 6 busy hours walking through the entire city, they finally catch up to him, and confront him and his associates - A fight inevitably breaks out. Now, the Wizard and the Cleric finally get to cast all those offensive spells they prepared in the morning (they may have used some utility spells, but they each still got at least 4-5 powerful combat spells). They have little reason to hold back,t hey are sure they have the culprit, and they need to stop him. So they'll use these spells, dealing a gazillion damage or whatever they can do with them. While the ROgue and the Fighter are just swinging their swords and contribute very little to the Fight.
We have a balance problem (Type 2), but not a problem for believability - that only one big fight might hapepn in this scenario is plausible, and one could very well argue that this wasn't a 15 minute adventuring day at all for Type 1 - the player characters were pretty busy, doing all that legwork. Time pressure in this scenario wouldn't work (at least not more than it already did). Wandering monsters? Well, maybe you could have put in some hostile gangs or some such, but it's not exactly likely that much threats would occur in a typical city if you're a band of adventurers. The only thing that may have cost the WIzard or Cleric their resources would be the use of heavy divination and enchantment spells - but they may not necessarily have needed them, or not prepared them. It could very well be that the only character that really would contribute the most in this scenario is either a Rogue or a Bard, thanks to their social skills, while the rest may have been mostly been the players thinking, but not necessarily character abilities. But that's not really the important part. The major aspect here is - the first few hours of the day were not sitting idle around, they were busy, the characters were doing all kinds of things, but these did not involve a major resource expenditure yet. But it lead eventually to a violent conflict.
And we can experience this even directly with wandering monsters during traveling, as well. THe party travels the whole day, and manages to not meet any wandering monsters, or evade them. But as they approach nightfall, their luck leaves them and they encounter a wandering monster and have to deal with it. They now know that they will go to rest soon, so it's inevitable that they will spend a large amount of their resources, knowing they'll be back soon (and that dead characters don't recover spells).
Or the 4E rogue might have snuck up done a boatload of damage action pointed done another boatload of damage and killed the murderer OH NO rogues are broken we need a more balanced game. We can go at this all day inventing bizarre scenarios that are essentially just corner cases. OH NO the wizard killed 12 minions and the rest of us mopped up after him killing the last guy omg so not balanced we must be his caddies right?