While I hesitate to re-enter this conversation which seems to be getting a bit "shouty", but I can see both sides of this debate.
As I said earlier, Mindjet obviously see their role as the leading mind mapping player as meaning that they don't have to worry about playing nicely with their competitors. And like a lot of software companies (Apple, Microsoft, Google, etc) they would love to create a "walled garden" around their products which makes it difficult for users to mix and match formats, programs and cloud storage systems across platforms.
Now I have criticised Mindjet and others for adopting this approach as being very short sighted. It can also be argued that Mindjet's dominance of mind mapping is not nearly as strong as it once was, but in their defence they still have the largest share of the mind mapping market, at least on the PC desktop.
The problem is that the rest of the market is so fragmented. This means that Mindjet were to make MindManager more "open" they would have to decide which of their many competitors' formats to provide import and export facilities for, and then keep these up to date as they and their competitors added or changed features. It is likely that Mindjet have considered the costs of doing this outweigh the benefits of attracting users from other mind mapping programs.
Also it's not like Mindjet's position on this is a new decision, a sudden downgrading foisted on unsuspecting users by Mindjet (yes they have form for that, but that is a subject for another time). I don't know about the early versions of MindManager, but at least since version 8 import and export options have always been very limited.
People might not like Mindjet's approach but they can't claim it's a surprise. And if you are planning to purchase a an expensive bit of new software with the intention of using it with a lot of files that you have created using another application, then it's your responsibility to find out if the new program can open these files before you buy. Criticising Mindjet over this is a bit like berating Subaru because you've suddenly discovered that the sports wheels from your old Ford won't fit the new Impresa you want to buy (or worse still, have already bought).
Like others I would like Mindjet to provide more import and export facilities, but we should have the same expectations of its competitors. At this stage they do seem to have it easier than Mindjet - it appears they only really need to cater for one file format. If they can import and export MindManager files, everyone seems satisfied.
It pains me to say this and will no doubt annoy a lot of people but it might be best to make the best out of this situation by turning the MindManager format into something a bit like the role that the MS Word, Excel and PowerPoint file formats have as the de facto established standard for office software. Even though Microsoft's market share is declining, an acid test of any new third party word processor, spreadsheet program or office suite from a competitor is still whether it can accurately read and write Word, Excel and/or other MS Office formats.
Adopting MindManager as the equivalent standard format, a lingua franca for mind mapping files, would go a long way towards resolving compatibility issues. True, Mindjet might end up not having to get off their backsides to address the issue but life would also be simpler for their competitors as well. They would need to provide only one format for importing and exporting files to make mind maps truly portable.