I've seen lots of recommendations for "just water and a few drops of soap" from a lot of places - including for cleaning my TV (with anti-glare coating).
Even when I follow directions exactly, there's always some kind of streaking, residue or uneven cleaning. Unless you also rinse or you use a rubber blade to clear the glass the soap will be left behind in some quantity.
I totally understand commercial window guys not buying retail chemical cleaners due to volume, but I'm not so sure that people frequenting detailing forums will be all that happy with the results. But hey, it's cheap to try, right?
I'd be more inclined to go cheap/green with vinegar or ammonia and water first. Although those formulas will have a little more cleaning power they lack any kind of lubrication so may also be annoying to work with in an automotive interior filled with awkward angles and long reaches for windshield and rear glass. Towels may tend to "stick" when trying to use it.
Honestly, for automotive use, I don't think I've even used one full $1.99 (or so) can of Bon Ami in over 5 years so cost wouldn't be a factor for me to switch.