To answer your questions:
1. This plugin allows the sprite font to be displayed correctly in the editor, as described in the first post. That is the only real benefit to using this over the native one that I can see and was the reason why I started to use it. I know that the array/JSON technique works just fine - as per the tutorials etc - I don't really understand why you asked the question and think you maybe missed the point of the plugin...?
2. It's not the end of the world for me, but it would have been if I had spent the last 3 months using this plugin throughout my project.... My point about 3rd party plugin support is about my concern that a critical plugin might get broken by a C2 update. If you've spent months on a project and then it's trashed by scirra's lack of support of these plugins - well, that's not a good thing! I am not saying that spritefont+ is necessarily one of these plugins, but this is just an example of the risk anyone takes by using a 3rd party plugin with C2. Of course multiplayer is going to be great...
1. I handle all text dynamically so I never see it in the editor anyway; that difference doesn't really affect in-game functionality so it's not something I'd considered - and that's why I asked the question .
2. I get what you're saying, and understand your frustration, but it's not really Scirra's job to make sure all 3rd-party plugins don't break. The same as it's not Unity/Epic/Adobe/Autodesk/etc.'s job to do the same w/ third-party plugins for their software. Again, I'd rather see them improve/support their own software than worry too much about those improvements breaking plugins created & supported by other developers.
It's just part of software development, regardless of the tools involved. And as you said, it's always a risk when using 3rd party plugins. I'm not especially thrilled when Adobe does an update that breaks my (paid, very expensive) After Effects plugins for my day job - and that happens a whole lot more than it does with C2 - but the companies who make them typically come out with an update shortly after. Luckily, there's nothing stopping me from continuing to use the old version of the Adobe software - or any of us from using a slightly older version of C2, considering 164 is a beta release anyway - until those updates are made.
TL;DR: It's better to be thrilled that such flexible, low-cost software exists and can be easily extended to make our lives easier. The occasional cost of using 3rd-party tools for increased functionality and productivity isn't limited to C2 and the benefits outweigh the downsides considerably.