So this relates to the tiered nature of such feats in 4e, how they increase by +1 additional bonus at each tier. Interesting. Maybe this is caused by D&D's insistence that all halves are rounded down, so the half-level bonus goes +0/+1/+0/+1 rather than +1/+0/+1/+0, but monsters get full level added to their stats? Anyway, I guess the question is really "Why didn't the designers just give everybody a stacking +1 to attack rolls and defenses at each tier?"
Well, if you start analyzing it, you'll find that monster defenses go up by 1 each level. This means that player attack rolls should also go up by 1 each level in order to hit an even-level foe on the same die roll. So, your to-hit bonus at level 30 needs to be 29 points higher than it was at level 1.
We already get +1 every other level thanks to the half-level bonus. That gives us 15 points.
We get a total of +6 from enhancement bonuses on weapons / implements or inherent bonuses if you use that system. Now we're at 21 points.
You get to raise your primary stat by a point at levels 4, 8, 11, 14, 18, 21, 24 and 28. That's +8 to your primary stat, which equals +4 to hit. Now we're at 25 points.
Epic destinies typically let you bump your primary stat by 2 points at 21st level, for another +1 to hit. Now we're at 26 points.
Where do the other 3 points come from? Expertise.
So, it's not really about rounding or anything, and yes, it could have been addressed by the rules just giving you +1 to hit at level 5, 15 and 25 or something like that. In the end, though, it's all a bit of a weird mask on the fact that you get +1 to hit each level (through a combination of stuff).
The same goes for defenses. (+15 half level, +6 enhancement, +4 stat boosts - although one will lag here, +1 epic destiny - two will lag here, +3 from feats)
I also didn't know that Improved Defenses was considered such a feat. I'll have to start taking it now... though I hate that it kind of nerfs the defense-specific feats, since it makes their +2's into +1's.
The defense-specific feats are great if you can take them! If you qualify for Superior Will, Superior Fortitude and Superior Reflexes, go ahead and take them. They're 1 better on the defenses themselves and they give you cool extra features. But you have to take 3 feats instead of just 1 with Improved Defenses (or else you let one or two of your defenses be significantly lower).
I didn't mean to imply that you have to specifically take Improved Defenses; I just meant that the feats that bump up your non-AC defenses (of which Improved Defenses is the simplest) are also a math fix to make sure that your defenses keep pace with monster attacks (which, like PC attacks, go up by 1 point per level).