> Asking for a phone number seems pretty valid.

I agree with your reasoning that for the vast majority of casual users it is useful (and maybe gmail is only intended for casual users). Otoh, there are countries where you could be punished for speaking your mind via en e-mail account that is linked to you (say by phone number) or from that e-mail account being linked to you via your using it to sign up for another service. For example, in the US one could conceivably be punished (or at least face an expensive court battle) for publishing certain types of software see e.g. https://en.wikipedia.org/wiki/Bernstein_v._United_States from an e-mail address linked to you