[Muromec]

It’s all fun and games until your bank reports something to the government under this SSN and it affects a person to whom its really assigned. Bonus points if “something“ has fiscal consequences for either of you.

Seriously, this is not kind of shenanigans you want to be doing for no reason at all, simply because the reason would be invented by someone else.

[gumby]

As my original comment says I don’t do this for anything tax-ish (for the same reason you mention). But there is no reason a phone company need know my SSN. Or the pharmacy.