I cringe when I see this written down. Along with "defiantly" for definitely and other examples. I don't see there is ever any excuse. And the excuse that "language evolves" doesn't wash, because "to of" is not a verb and never will be no matter how language evolves. It's not the same as adopting words from other cultures, new words like selfies, or a different meaning for "sick", for example.
OK, when we say "would've", "could've" etc, it may SOUND similar, but that's no excuse either. It implies the writer has never seen "would have" etc written down, therefore doesn't read much. What did they do at school? Was this style of writing used then and never corrected?
Another cringeworthy example is the inability to differentiate between there/they're/their and your/you're.
Just why do people do it and worse, why is it excused?