I admit I left myself open to this by responding to Creepers absolutist statement with my own opposing absolutist statement. Perhaps I should have said: if you can't think of "a reason" you must not be trying to
Your example is mostly an unfair comparison, because you're adding in the second line of to-hit bonuses to the AP total and then go on to discount to hit bonuses as irrelevant. So is Rogue Dili for 6 AP's a better deal than Halfling Guile at 10 AP's yes, but this just goes to prove that Halfling enhancement lines are pretty outdated and need to be looked over.
Of course this ignores that halflings can get that guile and some pretty good self healing, and saves bonuses, Hero's companion, extra dex, which are several nice things that make them great rogues, monks, healers or self sufficient pajama AC builds as well.
In the end the proof is in the pudding, Helfs are so good that you see one every once in a while in a pug, meanwhile there's 2-3 Dwarves and Halflings in just about every pug I join, along with a Horc or two, and the usual assortment of elfs and humans and WF.
To me Dili's are just a way to get part of what makes a splash powerful without getting MOST of the best benefits of a splash. Yes they can be useful to keep your capstone while getting a small amount of versatility. But again you're still missing out on most of what makes those splashes attractive to a build (to recaps that's class skills in UMD/intimi, lots of Skills that can actually make your character more enjoyable to play (like 4 spot so you don't literally slam into an enemy before you see them) feats, evasion, full wis to AC, and several useful first tier or early level enhancements).
I will also admit that when they either buff the race somewhat (adding Bastard Sword racial pref would do it for me I don't know about anyone else) or maybe reduce the price and then have 50% off sale on top of it I would buy them. And I'd do it so I could come up with some wacky triple class with a dili that made it play almost like a quad classed character (or a 3.5 rules complex PrE class build).