The thing with these tags is, you can always (or almost always) use a specific product/technology/language tag which would render the company-name tag completely redundant (see Appendix C for sloppy non-proof).
Another way of looking at this: how much useful information do you lose if you don't tag a question with a company name?
Note: I'm not talking about brand/company/organizations like wikipedia, amazon, flickr, facebook because those names also refer to websites/web services with which you can interact via API or from which you can extract data.
Appendix C: The "about" test
The relationship between a tag and a question can be expressed as such
This question is about [tag]
In case of microsoft,
This question is about [microsoft]
Fair enough. But in reality people are using these company-name tags like this
This question is about something developed by or related to [microsoft]
But if that's the case, you might as well just go with
This question is about [something developed by or related to microsoft]