The high amount of threat makes Mangle insanely better than any other ability we have. It is worth losing 1 ability and having dead time. Although, looking at logfiles abilities don't land at regular 1.5s intervals even when spamming them. Chances are you'll end up with less dead time. My Mangles are rarely exactly 6s apart as-is. While you don't end up waiting, you could be going beyond the cooldown instead.
You can't just assume cat or bear cycles will be the same then as they are now. Unless they drastically reduce Mangle's relative value it will still be worth using every cooldown and lowering that time at any cost. It sounds like they are only increasing it value. Depending on changes to Swipe bonuses (like 4t6), it might not be worth using on single targets anymore. In addition, think of limited threat situations where you can't spam 4 abilities every cycle anyway. We'll need to get all the numbers in to determine how much changes to Maul will impact its usefulness too. It might end up being better to spam Mangle/Maul and just Lacerate in between.
A shorter cooldown also means a higher uptime percentage of the debuff as well as any procs that might be associated with it later. A 6s cooldown and 12s duration means missing 2 in a row the debuff will be off at least 6s more. A 4.8s cooldown means missing 2 in a row the debuff will only be off 2.4s more. Hell, it might even be better to not take all the points. Think of only taking 2 points (5.16s x2 = 10.32s) to line up the cooldown so that you can Mangle again just as an
[Idol of Terror] proc is wearing off.
Future itemization and exact threat number changes will change the value greatly. However, unless something new is introduced to make keeping a longer cooldown a good thing, I can't imagine not taking the talent.