I'd expect it to work flawlessly with Max and Maya as they're Autodesk.
I do not agree. XSI and Lightwave are not Autodesk products and work flawlessly with FBX. When an exporter doesn't work and other external developers can make excellent exporters, you have to admit that you can't blame the format except maybe for the docs.
developers are effectively only using a small subset of all the features available anyway
Only if by 'subset' you mean program specific features (which any modeller handles differently), because most of the important stuff is very well imported (that is boned and morph animations, maps and lights). Anyway, this just makes the job easier, since Blitz Research can choose what to export and just avoid the rest.
I'm sure Mark will have his own thoughts on which format he will concentrate on.
I sincerely hope so, because with B3D he avoided the issue entirely after releasing the format... however, at the time of the release of B3D this issue wasn't as pressing as it is now.
Anyway, I can't see a single reason because supporting either one or two well known formats (forcing everyone else out) is better than FBX when FBX works exceptionally well with well known formats. And you just have to keep track of one format instead of all of them.
The minority would be someone which wants a direct exporter for his product. However, it makes more sense to have a common solution and then having others writing direct exporters to the proprietary format if for any reason they want to do so, than having everyone to write the exporter for his own format (which, being a daunting task, often results in limited exporters which are far worse than an official one) because only a bunch are supported at the discretion of Blitz Research. Of course to have also some direct exporters would be good, but not at the expense of FBX.