on the arpeggios vs scale issue, it's more lile chords vs scale issue, since appregios is simply the chord notes broken up into individuals played in a sequence.
you defintiely can play scales, but the notes of the chords are the important notes to play in order to provide the sound of the key you're playing. If you play the notes in the scale that are not the notes in the chords, it may not sound like the key at all.
So if you want to underline the chord changes in the song, you'll need to play the notes of the chords.
But most of the time if you play a chord on the bass it'll be too boomy and too strong for a suppoprting role. you definitely can play chords but if you play chords all the time with the bass, it'll be very tiring to listen to.
so you break them out into appregios.... moreover when they are borken up into appregios it becomes more interesting, there are so many variations and sequences you can play from those chord notes, up to you to play.
but of course, I take these as my own guidelines and definitely not set in stone... they are meant to be a guide and definitely meant to be broken too! nothing should come in the way of creativity and innovation.