Instrument assignment is mandatory.

Categories are very basic modeled after GM standard because that was readily available. They can be extended easily. But custom items don't help because they are not portable. Your custom category won't work in my studio and vice versa. The whole point of a category is to make sounds replaceable.

Many vendors use tags which are great for browsing but complicated for comparing sounds.

Which categories do you miss?

That's what I though at first too. It almost made it into the product. But the categories in Synfire are not for browsing convenience. They are a system to automatically find replacement sounds and to share arrangements between studios. For this to work every installation of Synfire needs to use the same categories.

It easy to add (many) more of them though. If you want to suggest some ...

If you saw a printed score that said orchestra for every instrument, how would you know what instrument to pick for each of the tracks that would sound something like the one the composer had in mind? Likewise if it said Omnisphere, how would you know which of the 41000 patches each track should use.
That is a name of a product or a sample pack not a category. A track played by omnisphere could be defined as a piano, strings, bass etc, which gives a better idea of what type of sound is used. As andre said, it also allows someone who downloads a cognitone track who doesnt have omnisphere, the ability to pick a sound that has a similar 'flavour' to the one in the original tune.

That's articulations not sounds

The purpose of instrument category is to tell what kind of instrument the sound is. Every sound library uses different articulations

You are still missing the difference between an instrument and a category. You can name an instrument anything you like, I have some pretty unique instrument names (eg MyWizzBangFiz) for various synth patches I've created myself. Nothing in the name gives synfire any idea what it should be used for, what the playing range is, or what other instruments can be used instead if that synth (or vst/bank) is not available. So I assign it to a suitable synfire category. The instruments in a category should share some common sound characteristics, not that they all use the same sound vst.

You can still create a rack and fill it with any instruments you have access too, and call those instruments what ever you want, it doesn't have to match the same name as a vst preset as long as the device description knows how to select the preset. So you are free to call all your instruments Kontakt that is really what you want regardless of whether they are a kind of piano, bass, lead synth, violin, etc. However if you assign a suitable category to each sound, synfire will do it's best to assign the correct playing range and other capabilities to each of the sounds.

Cognitone have said they are open to adding additional categories and asked you for suggestions. For a category to be useful there needs to be a 'family' of sounds that share something in common with each other sound wise that makes them different to those already present. Your suggestions so far are not suitable, even you pointed out that one of your suggestions, orchestra, contains multiple different types of instruments, other suggestions such as The score, omnisphere, etc suffer from the same problem. I'm sure you would be the first  to complain if you previewed a cello figure and it played it with a piccolo because you currently had a piccolo sound loaded (both are orchestral instruments).

Whats missing is several types of flutes and other ochestral instruments, for example