IMHO, if one has a relatively clean and non abused engine then Kreen really doesn't need to be used. I do know and have read where people have used Kreen to clean up stuck piston oil rings and for semi to severely neglected engines with good success.
BG MOA is good stuff, a friction modifier to some degree and really can't harm much. The only time I wouldn't use BG MOA, Liquid Moly MOS2 or the like is in an auto that sat for long periods of time. Otherwise really only a hit to the wallet is its only drawback.
Whenever I use a FI cleaner (have had some great success with Royal Purple lately), I use it right before doing an oil change, and give the auto an Italian tune up.