Are we talking about asking or requiring ? I agree no one should require a phone number, but it's completely reasonable to ask if people want to use that rather than email.
Agreed, but at some point asking starts to feel like badgering.
Basically every time I log on to Zipcar.com to make a reservation they throw up an interstitial asking for my phone number. It gets old. If I declined the first 5 times, what will make the next 50 times any different?