Now, as for the 3.5 crowd . . . yes, it's true that a Cleric can use his Spontaneous Casting class feature to use cure or inflict spells (depending on his or her alignment) whenever they want to by burning one of their other prepared spells. However, there are still a few things to consider.
This doesn't work with domain spells. If you have a domain that allows access to a healing spell, it might be for the best if you choose that one, especially if you're expecting some trouble later on in the game. After all, what are you going to do when you're all out of regular spells, your main fighter is three-quarters dead, and all you've got is a lame Heat Metal to your name and there's no way to change it to a Cure Moderate Wounds?
Eventually, some of those spells are going to have to be used up as cure spells. Adventurers get hurt. It's a basic inevitability. There's little point in not setting some of your repetoire as cure spells, because that's what they're going to end up as anyway.
Some players can go to extremes concerning this feature. They may end up thinking "Oh, I can cast this offensive spell, no problem, 'cause I can burn one of the others later if I have to!" right up until they have only two spells left, neither of which they truly want - or can afford - to change into a cure spell. On the other end of the spectrum, they may end up burning one spell after another into cures, and end up with little to no offensive or defensive spell capability at all. Selecting a balanced roster of spells in the first place can help keep them mindful of that balance.
The resulting cure or inflict spell must be of the same level or lower than the burnt spell. It's a pain in the ass to use up all of one's third to sixth level spells as offensive spells, then only have a handful of Cure Moderate Wounds and lower to try and heal up a couple hundred points of your team's cumulative damage.
So, nah, I wouldn't exactly call it a waste.
