Exploring is great and all, there's some real nice stuff to find in vanilla Minecraft; however, if one is more of a building mindset, then stumbling on a motherload of gold is great and all... right up until you have jack all to do with it. At that point it's just finding a rare stack of blocks that are more useless than cobblestone. These mods aren't really aimed, from what I can tell, at significantly improving the exploring side of Minecraft so much as the building side of it.
For me, in vanilla, the part that really shines is digging about, exploring underground, right up until you have more iron that you know what to do with, unless you want to build a huge rail system, and you get a handful of diamonds. It's a classic, lovely Skinner box. You hammer on the lever (mine/explore), and occasionally, if you do that, you get treats (iron/redstone/gold/diamonds), and if you're not on the ball, you'll occasionally get punished (creepers). Once you've got a decent stockpile of those treats, they start becoming devalued, and the system starts to fall apart.
That's not to say there aren't rewarding things to do. For example, you can build a grand structure; this is something that will require many resources, and be quite the accomplishment. Unfortunately, most of what it requires is the bog standard resources, such as stone, sand, dirt, wood, or even such things as netherrack. They're abundant and not at all hard to find. Relative to searching for that other stuff, it's an insignificantly rewarding experience. Granted, in the end, you do get to construct a grand... construction, which is quite rewarding, but the more grand your construction is, the more hours upon hours of fairly minuscule rewards you have to slog through to get there. Sometimes it's just not worth it.
These mods, on the other hand, or at least what I've used of them, keep the worthwhileness of the rarer resources up as they have many uses, and make the tedious to gather things much less tedious to gather, and as the cherry on top, they give you all kinds of gubbins to build and play with that result in a much nicer reward schedule while playing the game. That's how I see it, anyway.