I wouldn't. Most residential phones are moving to VOIP; if you have Internet it dramatically cuts your phone bill.
You don't have to. But it would be nice to have the choice.
The "Callers'" number can be blocked, and some unreputable robocallers do that. But, then I block all calls that don't identify themselves via Caller ID.
Which is close to pointless, since most robocallers just use a fake caller ID number anyway.