I have researched and read that there are outlook backup vendors that
offer incremental backup of .pst files.

Is this really true? AFAIK, .pst files change so inconsistently that
even deleting a message or moving it to a different folder wipes out
any kind of "clean" incremental change to the pst file (i.e. the
"delta" in the PST file is hard to detect "cleanly").

Also, when you archive some emails, the "new" PST file is practically brand new.

Please shed light on what is fact and what is myth.

the incremental backups are intended for Exchange Servers, and not end
users. I save mine WHOLE to DVD quarterly.