Sure. But we are talking about skill checks where one guy can't get a success for the entire party. It is a question of what you would rather have. Lets say you take the 13, and as a result you find one obstacle trivial, but then you are stuck with a +0 on the check for the skill you didn't take any ranks in. This is fine, there isn't anything wrong with it. But I have had plenty of characters be better off taking the two +7s. So that they a fair chance of overcoming both checks.
It really depends what you set the DCs at. As someone else mentioned, if you use opposed checks against any monster with a CR close to theirs, they are all going to fail except for the one who has maxed out ranks.
If you set the DC to 20, then it is fine having +7. But it's going to be extremely easy for the 11th level guy with a +5 modifier in the stat. So, the question is, are you going to plan around the fact that the guy with the high modifier will be there?
I think this depends heavily on how often the group splits up. Mine never does. I've been playing Living Greyhawk in 3.5e and Living Forgotten Realms in 4e. I've played in 8 or 9 different cities now with strangers from various locations. I've gotten a good cross section of playstyles and learned things I like and don't like about the way other people play. However, one constant remains, and that's that NO group I've ever played with splits up. Even in city adventures. We work together. The person with the +27 Diplomacy is always in the group and is always the one making the check. Anyone who doesn't work WITH the party tends to be berated with not being a team player. The feeling is "we are all here to play the game together, we all want to accomplish our goal, the point of the game is for us all to contribute our individual skills to the cause". This seems to be the major difference between how I think of the situation and how you do.
There's a number of reasons we don't ever split up, most of which I made in a previous post(mostly, it involves not wanting to sit around and watch other people play for 1-2 hours before the DM gets back to you).
I have not just made the assertion, I have built a case for my position. And I have managed to do it without being insulting. You disagree, and maybe in your games the spread doesn't work, because you rely more heavily on team work.
I don't believe you built the case that half ranks are worthwhile. You made the case that half ranks are worthwhile in a small subset of circumstances.
For instance, is it worthwhile to have +7 to your skill check in these situations:
1) The DC of the check is 28 or higher.
2) It doesn't matter who uses the skill, your entire party is together, someone in the group has +30, and he is willing to use the skill.
3) Someone has a spell that allows the entire group to bypass the check and there is no need to use the spell for anything else in a day(for instance, Mass Fly to avoid jumping over the pit and there is no need to fly for the rest of the day)
4) The DM gives you the benefits of succeeding even if you fail.
Mathematically, it just doesn't matter at all in these situations. It matters when:
1) The highest skill check character is unavailable(or something prevents him from using his skill), the DC is lower than 28, and no one has a spell to bypass the skill check.
To me, this seems a small subset of "times a skill check will be made in all styles of D&D game". A very, very small subset. It may work in your game, but understand that your game appears to work very different from the average game, from what I've heard.
On the other hand, even when the biggest difference between skill checks happens in 4e, there's still a minor chance to succeed. For instance, if you start with a 8 the stat for a skill and never increase it or train in the skill, then at level 30, you have +15 to the roll. If someone starts with a 20 in the stat and increases it every chance they get(including taking Demigod as their Epic Destiny for another +2) and train in the skill, and have skill focus, they have +33. If the DC is 35, you still have a chance of succeeding and the expert still has a chance to fail. The DM can set that as a DC and know that there is a reason to roll the die because the party MIGHT fail. And everyone should roll the die because they have a chance of contributing. It also still lets the person who is good at the skill shine.
And this is in the most extreme case. Most of the time, the numbers will be closer together than that. If no one has a stat quite that maxed out and no one took skill focus, the difference should be closer to 10. When the DC is 35 and the lowest is +16 and the highest is +26, there is a real sense of "I can still help out the group here. I still have a 10% chance."