From another thread, but very useful here!

QUOTE
The modes ARE different. If you play only the white notes, C major scale, you are probably in the the mode you are in depends on the start note (D=Dorian, A=Aeolian), and a choice of 5 others).
The major key is just one of the seven possible scales/modes.

As a general rule, in neither case should you use a G7 chord, but a G instead, because the G7 contains notes which the ear expects to resolve to a C chord at the end of a phrase. Dominant 7th chords are suited to major and(harmonic) minor modes only, for which they were invented.
UNQUOTE

Ha! That's the technical reason that the Stradella bass 'fakes it' so well when you have a 'small bass' box. You can fake it with the Basic chord, and not the 7th version of that chord - the small boxes don't HAVE the 7ths anyway, but if someone somewhere in the group is playing the 7th as part of the melody, you have what the ear needs anyway! What you (the P/A player) are doing is playing the basic 'modal chord' which is the underlying basis of the 'Maj/Min structure' - these are just 'extensions' to word it one way! If the 'resolution' is needed/expected, or not! You're letting the others do the hard work playing the fancy bits!