I strongly agree with Flimm on keeping them distinct.
My opinion on mobile is that it's overly broad and ambiguous being a general adjective. That being another discussion, see this: Can we please disambiguate or discourage the 'mobile' tag?
A specific tag like ubuntu-for-phones is a lot more self-descriptive and much less likely to be used erroneously. Especially for new users. I'm not saying this is a good tag name, though.
Clearly, this is likely to result in a lot of retagging work ahead in reviewing questions if we would merge them. This is not gonna be fun and may discourage people from reviewing. Now is the time to prevent that from happening.
Other supporting arguments against merging:
- Canonical's ambitions are huge on this product. If Landscape, Ubuntu One and MAAS deserve their own tag, then it does not make sense to me if 'Ubuntu for phones' would not.
- We can merge later. IMHO just one more tag is very harmless as it can be merged later if we decide to do so.
- We don't know much yet how 'Ubuntu for phones' is going to work out. See the response to David Planella's comment below.
- As pointed out by Flimm very well in his comment:
It's easy to subscribe to more than one tag, but it's impossible to subscribe to half a merged tag.
In response to the answer of Jorge Castro about current subscriptions of the development team...
In my opinion the team(s) are just subscribing to the wrong tag here. It is to their advantage to have a distinct and obvious tag for a such a product they're working on.
In response to David Planella on "It's the same OS, just different devices" (paraphrased):
Canonical may present this to be as the same Ubuntu for users, but it just can't be the same. This is the marketing machine and it's working well apparently. Regarding Q&A this will be horrible, I'm expecting. One does not install a phone using a ISO. One does not want to receive software recommendations for Desktop-UI apps. One does not want to read answers about shortkeys on a touch screen phone. We don't know yet to what extent it will be the same to the desktop OS in terms of technical aspects, release management, customizations by operators, etc. All we know for sure is that it will have a different UI to users, a completely new approach regarding app development and it will simply result in different questions/answers.