If somebody is doing that, they are doing something very weird.
And I will gladly accept I am doing weird things, as many people (and I have no reason to doubt their wisdom) consider me a weird person - but all of my GPS navigation has been based on coordinates I typed into spreadsheet software by hand, or, in rare cases, found on the Internet (basulm.fr is one such source)
OTOH it is of course clean daft to depart on a route, GPS or not, whether flying or driving or snorkeling or whatever, without having checked it against every possible reference. But I have at least as much mistrust/reserve/caution against commercially bought stuff as against my own typing.
Even the official VFR charts for my own country, published by our famous National Geographic Institute, have a strong tradition of containing at least one error - I feel slightly uncomfortable for not having located any in this year's edition, as yet.