Hey all, since I'm the founder of codebender, I thought it would be helpful if I pitched in as well
First of, Paul is definitely right that Arduino.cc are making Create for Arduino.cc and Arduino at Heart boards, in fact they've mentioned this somewhere, and it makes a lot of sense. They've found themselves in a spot where they are the IBM PC, everyone else makes PCs, and the average consumer doesn't really care, know, or care to know about the manufacturers.
Having both IDEs allows them to be open, but get as many people as possible using their walled garden, and offer them reasons to buy the official boards. Add to that that they want to get into IoT, and compete with services like Xively, PubNub etc, and one can easily understand where Create fits and how it's an important part of the new "Arduino ecosystem" that Arduino is building. And definitely, the whole split with .org/.cc expedited this for sure.
The problem is, building a Cloud IDE is a terribly hard thing to do (trust me, I know), which is why Arduino has been talking about this for 4 years, announced it 2-something years ago, and it's about a year behind schedule AFAICT.
There's been some talk about supporting Teensy on codebender, and honestly, that's exactly our business model and our differentiation to Create. We want, and depend on, supporting as many devices from as many manufacturers as possible, since we don't make sales from our own hardware, but from the people who upgrade to a Pro version, and sometimes from the manufacturers themselves. The tech still needs some work, and we will need to talk more with Paul, but we'd love to do this
As for the Cloud VS Desktop IDE, both have their advantages, nothing is a one-stop-shop solution. If you ask people 10 years ago noone would want to have a Cloud-based (ok, Web-based since the "Cloud" buzzword didn't exist then) email client, then Hotmail, Yahoo! and GMail turned this around. Same for GitHub, and every other productivity tool.
The "Desktop IDEs are better, Cloud IDEs suck" is a fallacy. It's all about the implementation. It's definitely easier to build advanced Desktop Apps since there's so much technology and you're not limited by the Web sandbox, but compare Word and GDocs, and I'd go for GDocs every single day, the thing is just AMAZING. Having said that, not everyone cares for collaborative editing, history/revisions, and accessing their Documents from everywhere, if you care about word processing features and compatibility, Word rules.
So, in my slightly biased opinion, the reasons to use a Cloud IDE are having a shared workspace for the whole team to use, having a defined & shared set of libraries for the whole team to use, same for the compiler/toolchain including tools & specific versions of these tools (let's call all these "having the same, shared environment across the team"), working from multiple computers and switching between them, collaboration, multi-OS compatibility with minimum development effort & without the OS-specific caveats, minimal setup & configuration costs, easy/auto updates. And depending on who you trust more, yourself & your Operating System's vendor, or your Cloud IDE provider, security & data loss/storage/backups.
So as people mentioned, a Cloud IDE is perfect for education where everyone involved (students, teachers, IT department) want to deal with as little stuff as possible. And then for the hobbyist/newbie who just want the least amount of effort required to get started. But also for the Professional who doesn't want to deal with his tools, & maintain and update them, he'd just rather pay someone to get this done and focus on code
And the main disadvantage? Yep, lack of "if you can't open it you don't own it" hackability, and if the internet is down, so are you