Why do the 7 note minor and major scales sound bad compared to the pentatonic versions?

What a strange thing to say - I've never thought of some scales being nicer than others. You seem a little confused though, are you talking about major and minor scales or modes?

Also, what is the "big fuss" you've come across about modes? I guess I do lead a fairly quiet life but I do try to keep up with things and I've not noticed a fuss. Why is there no big fuss made about the positions of the pentatonic scale...? I guess because people have lives and more interesting/important things to think about!

If, as you say, the only thing you like about the regular (major?) scale is the leading note, you're going to be playing some rather dull bass lines.

This is the first time I've heard anyone say something like this, and I know a lot of musicians with both a good ear and theoretical knowledge.

Are you sure you are playing and using them right? For example, if you use a backing track with chords, you can't just play all the scales over it. If you drop the 4th and the 7th in a minor scale, you're not playing the pentatonic minor scale. And, for example, the 4th in a major scale is an 'avoid note', meaning it's a semitone above a chord note, and therefore is often only used as a passing note. Even the genre is important. A majority of punk songs probably require the minor pentatonic over other minor scales.

I'm a bass player myself, and I agree that it's fun to play pentatonic bass lines, you can get really creative and write cool riffs and melodies. But when I play an entire set with a band using only pentatonic scales, it'll get really boring. I need to use entire scales, even use chromatic lines outside the scale, because that opens it up and makes it interesting.

For example: using the 9 (the 2nd note an octave higher) in a minor scale can make people go 'wow' (if used sparingly and in the right place, off course). On the other hand, using the major 7th (again, only sparingly and the right place) in a major tune can really open things up.

How, when and in which context you use your scales is really important. There's nothing wrong with them when you just play the scale by itself.

I agree with the previous answers. This is a strange claim.