Celebrim said:
Your point is only valid when you spend the feat to improve something that you don't intend to invest anything else in AND the skill is one where the challenges tend to scale with level.
Exactly. And if I use a feat to boost a particular skill (or two), that skill must be pretty central to my character concept. So it's pretty sure I'm also going to be buying skill ranks as I go up. So I'm generally interested in the DCs that, for my particular skill level, are neither automatic success nor automatic failure. For any task in a very wide range (a spread of 19 DCs) +2 always increases the chance of success by 10%.
As to the original question: What good are these feats? It depends on the pair of skills. Some of them are really natural pairs, like Stealthy or Alertness. It's not hard to imagine a character concept for which both Spot and Listen are central, or both Hide and Move Silently. In fact, in each case, a character putting a lot of skill points into one of those skills usually also puts a lot into its companion. For others, like Magical Aptitude, the combination is just silly. Seriously, what kind of character wants to develop both Spellcraft and UMD? Spellcraft is best used by the kind of class that doesn't need UMD. There's a reason there isn't a single class (well, core class at least -- although I'd be surprised if anyone's developed a non-core class with both) that has both of those as class skills.
The folks at WotC evidently decided that all skills should be paired up, and had to be very creative to cover them all. Well, almost all. They didn't even try to include the various Craft, Perform, Profession and Knowledge skills, and Concentration got left out, the victim of Odd Number Disease. The might as well have thrown up their hands and left UMD out -- matching it to Spellcraft really doesn't make sense.