Just pragmatically speaking from System76's point of view, and other partners of Canonical; I would imagine that it would be in their interest to have an account which was subscribed to a system76 tag, so they could offer "Official Answers" to questions regarding their service. This would likely help them streamline their support, instead of relying on ubuntu forums.
I think that if there are people who work for System76 (to use that brand as an example) who intend to provide support on Ask Ubuntu in that official capacity, and they know how Ask Ubuntu works and will comply with our community's norms (in terms of our standards for good answers, for example), and they have a significant preference for a system76 tag to exist and believe it will help them help people here, that then we should strongly consider allowing such a tag to be created.
In that situation, I think your reasoning about helping the company provide support has merit. I do not think we should have these tags in anticipation that they might help System76 employees provide quality support on Ask Ubuntu.
I'm don't think their status as an official Canonical partner per se is the main reason why this situation would be special. I think anytime a brand's name is mostly synonymous with a particular product or product line and there's no other, separate, commonly recognized name for the product/series, and an organized group of people (not even necessarily the company's employees) can explain why having a tag for it would substantially enhance their ability to contribute to Ask Ubuntu with quality answers, I think most of the same considerations apply.
As for whether or not we should make a System76 tag right now: Unless you're a System76 employee or similar and you are trying to tell us that having such a tag would substantially improve your productivity in creating quality answers for your product, I think the answer is no.
I do agree with Eliah's answer, I just wanted to stew it down:
We would love companies to support their Ubuntu users on Ask Ubuntu. That gives us answers from professionals and helps the wider Ubuntu community.
They can have tags to help them. Most of us take a fairly utilitarian approach to tagging and can abide whatever makes most sense at the time. If that means having a branded tag, so be it.
But we shouldn't give anybody a tag. This has to be a process that the company wants to get involved in. By asking for a tag like that you'd be committing to maintaining its questions. One of the SE Community Managers posted some best practices for a similar problem on Stack Overflow and I'd say those broadly apply here too.
So if you represent a company and you want to commit to answering questions about your brand or product, either drop us a Meta post or use the contact link in the footer to talk to SE directly.