I guess it may depend upon what someone would categorize as "affiliate income". When the proportion of some bloggers' revenue shifted to bank card referral revenue being the plurality of the revenue (and income), some bloggers getting more serious about the business -- and also tax-adjusted personal income outcomes (and perhaps liability concerns) -- would be the natural course of things for those who came in with very limited (or perhaps even no) business/entrepreneurial experience and then found themselves with a business and not just a hobby with hobby income.

Some bloggers got into setting up LLCs for their blogs as the $$$$ signs filled the eyes, courtesy of revenue coming in from bank card referrals and other advertising/marketing/affinity partnerships sending money their way.