RAI the mantle was clearly meant to make it normal transparency. However, that last sentence doesn't effect the cheap interpretation in the slightest. While having complete transparency is insanely useful in normal transparency games, it is just a bit more useful in non-transparency games. As for the first part of the sentence, who is to say what most other games treat psionics and magic as, you only know about the games you've heard of.
You're right that, relatively speaking, the majority of its usefulness in a non-transparency campaign comes from forcing normal transparency, regardless of uber-transparency. That has nothing to do with it, though; the bold text makes it pretty clear that this doesn't have any effect on "most games", and because there's no indication in the XPH of the game EVER working the way people seem to be interpreting the Magic Mantle, it doesn't matter what, precisely, I think is the most common of the rulesets presented there. Since the XPH never suggests that you can apply metamagic feats to powers (as my favorite example of a silly thing to do), that is almost certainly not how most games work, so it can't be what the Magic Mantle does, because otherwise it wouldn't be redundant.
Who is to say what most games use as a ruleset? For all you know, every game but what you have heard of plays with non-transparency.
It doesn't matter what I know about it. The developers set Magic-Psionic transparency as the default, and as a result that's almost certainly what THEY believe to be the most common (and thus what they were referring to when writing the rule). Even were that not the case, it's preposterous to suggest that most games are run in a way not even written in the books, which is what you have to do when you say, "I treat magic and psionics as identical, so I'm gonna apply Persist Spell to this power!"
It doesn't matter what they believe to be the most common or what they intended a sentence to mean, it's what is written that matters.
To an extent, you're right; we can't always know the designers' intent, and we have to make do with what we have. But when the developers have made CLEAR what the intent was, as they have in this case, ignoring it is like claiming that a Vigilante has 20 3rd level spells. If the sentence I bolded in the first post didn't exist, then I would have to agree with you and say, "This is a rule they didn't think through very well, and we can abuse it." But they went on to print a clarifying sentence which is sufficiently clear unless you're willfully trying to misinterpret the rule.
Also, it is not what you are stating when you use complete transparency, it simply is what the developers stated with their badly written rules.
But it IS what I stated when you use complete transparency. I might have worded it in such a way as to make it seem absurd, but that is actually the argument: "Psionics is equivalent to magic, so I can apply metamagic feats to powers." Even if you could, it's like applying Extend Spell to Alchemist's Fire; you can sorta see how it MIGHT work (double the duration of burning), but it still doesn't really make any sense because Extend Spell refers to all sorts of mechanics that don't exist, like Spell Level (even if the adjustment is 0, it still exists as a mechanic).
Also, claiming that different groups play different ways is kind of irrelevant. I'm not arguing that following the rules 100% is what anyone does. I'm arguing that the only reason somebody would play this way is if they despised balance (which is fine, but you probably shouldn't suggest this trick to anyone else or hope to use it in a build meant for anything but a theoretical exercise), because it's not in the rules and it breaks the system into itty-bitty pieces (well, more than it was already in). If you want to make a "But casting has ridiculous trick X!" argument, great, ban trick X because it's breaking your game. Don't use it as an excuse to break a much less absurd system by deliberately misinterpreting a rule and ignoring the clarification printed in the following sentence.