Except as pointed out that's generally not sufficient for Japanese since given their kanji based name you'll have no idea how to pronounce or sort it without them also telling you in a separate field how it's pronounced.

There's also the issue of interacting with other systems. You may want just one field but some credit card processor might want 2 etc....

Except in this case your "Dear <insert full name here>" e-mails might not sound very friendly or appropriate.