It's a dps gain, but not a huge loss if you're forced to switch to mangle.
Shred is NOT a dps gain over mangle in any measurable quantity over the course of a boss fight.
As Aggixx said earlier, shred is only a dps increase in two scenarios:
a.) You'll energy cap, might as well use the ability that costs more.
b.) It lets you know when you're behind the target (or rather, it tells you that you're in front and won't let you use the spell.)
Mangle is a dps gain or a wash in situations not involving the two above. (In either case, we're talking tenths and hundredths of a single percentage point, akin to being 3 rating or 6 rating over the hit cap.)
If you're going to argue that to a well-played druid won't need the glyph of shred (hah), then I can equally argue that neither of the above two scenarios exist for this imaginary perfect druid. No one plays perfectly or even close to perfectly enough to always be able to shred and then mangle if he can't. There's many a time when I've noobed it up when I hit my shred and then dodged some mechanic only to realize my spell didn't go off and I energed capped like a boss while I wasn't paying attention. I'm not trying to say you suck, I'm saying that we all suck and it's ridiculous to assume perfect play. I'm one of the top dps in one of the top guilds, and if you care to examine any of the kill videos linked in my sig, you'll see that I still make rotational errors at an atrocious rate. For example, on our first Garajal kill which was nothing more than a pure 6 minute patchwerk, I think I counted 35 rotational errors that I committed. Get into something more complex like Will or Sha? I'm thinking literally hundreds of mistakes over the course of the kill.
What it comes down to is that there is no measurable difference whether you use mangle or shred. Shred has some benefits over mangle. Mangle has some benefits over shred. These benefits are both somewhat intangible so you can pick whether you like a "get behind the target" alert or a "I can dps from whichever way I'm facing" advantage. Or heck, interweave both. (I happen to be of the persuasion that aside from fights with no secondary targets - like 3/16? this tier - mangle is a real-world dps increase.)
If you need a glyph slot and don't want to use shred, mangle is perfectly fine.
I think after all of the years where people were ridiculed for suggesting mangle over shred has really gotten people to scoff at the idea and not really even entertain it.
Personally I love using mangle when I know I'll be running into a situation where I'll need combo points and won't have the energy to sustain what I'll need with shred. Being able to use that finisher a second or two quicker can be more than enough reason to allow for mangle usage.
Last edited by Dysheki; 2013-02-27 at 02:22 PM. Reason: grammarz
That said, if you really feel the need to free up that glyph slot - do it, and just mangle when you're in a situation where you would have been utilizing the glyph.