It's information DMs need and players shouldn't have to bother with; also a poor side effect is that too much other information is sometimes revealed to players who do the math e.g. creature's AC, + bonus on that new magic sword, etc.
What???
A player doesn't need to know every magical item out there, just like he doesn't need to know every spell out there. He does, however, need to know what the ones his character has do. You'll note that every pre-4e PHB devoted over 1/3 of its pages to spells.
Overgenalizing for effect. Also 4e-style monster (and NPC) stat blocks generally don't break out the effect of items they carry individually; they're part of the monster's abilities. So you don't need to know a magic sword's stats when a monster or NPC has it; it's only when it moves into a player's hands that you do.
In previous editions, did you ever have trouble working out what your character's handful of magical items did? Our group never has but your experiences may be different. Spells however were a high-use character feature for magic-users and so players definitely needed access to this information. As such, I'm still not seeing the necessity here for putting them in the PHB. There's enough in the PHB anyway, without further burdening it with a chunk of pages of non-essential campaign specific information.
Exactly how much can fit in a bag of holding? How many people can fit on the carpet of flying or the folding boat? What does a belt of giant strength do? Obviously a +1 flametongue sword has something to do with fire, but what exactly are the mechanics (especially pre-3.x when the effects of that kind of thing were largely standardized)? And I grabbed the DM's copy of the DMG when I was playing 2e (because I was a broke college student and didn't have my own), or in 3.x brought my own copy. Of what was supposed to be a DM's book, but I had a copy anyway even though I never ran a game in person (in other annoying multi-book stuff, stats for summoned animals and common riding animals -- yup, in the monster manual, which players are even less likely to have than the DMG).
Magic items are essential unless WotC wants to radically depart from prior norms for D&D or wants broken math. And only the barest handful of them are campaign-specific.
Normally the DM tells you. Although, imagine the DM didn't. Imagine your character had to work out the capacity of a bag of holding, or perhaps you could roleplay that your character thought it could hold an infinite amount. Perhaps you could squeeze four people on that carpet but then when it flew off, only two were able to magically hold on (with the others falling off). It opens up a little more imagination and discovery than having an ironclad this item does x.
I think a lot of people were complaining about the christmas tree effect in 3.x while others were complaining how anaemic MIs were in 4e. Anything that tones down the reliance on magic is a good thing in my opinion; let the heroic actions of the PCs come to the fore rather than their suite of undeserved and spoonfed MIs.
