OK, to expound on this...
Modes are useful if you're playing modal music. If you're playing chordal music they might lead (though through a convoluted process) to the "good" notes, but it's a terrible way to get there. So before you start worrying about learning modes, make sure you OWN basic harmony theory. Our job as bassists is to both connect the rhythm with everything else going on musically and TO DEFINE THE HARMONY. In order to fulfill the second part of that job, we have to understand the harmony.
So, start with owning the diatonic major scale- know how to build it, how to figure out the correct notes (including correct enharmonics) in any key, what it sounds like, and how to physically play it smoothly in any key up and down the neck over at least two octaves. It's simply not enough to just be able to play it in a couple of positions, you need to be able to see it and know it in any key anywhere on the neck.
Then, learn the basic chords- if you don't know that a Dmin7 is D F A C, how can you define the harmony for the rest of the band? That's critical. Know the major, minor, augmented, and diminished triads. Know the 4-note chords (major 7, minor 7, dominant 7, minor 7 b5, and diminished chords).
Then go back to the major scale, and learn the logic of the harmonized scale. It's not enough (nor really useful) to simply know that the I and IV are major 7, the ii, iii, and vi are minor 7, the V is dominant 7, and the vii is a minor 7 b5 (a/k/a half-diminished). You need to understand WHY and how those were all derived from stacking the same scale up in thirds.
When you have all this, then the modes will not only make more sense to you, they'll be easier to understand, and most importantly you'll be able to see where they're useful and where they're NOT.
See, the common practice of someone telling you to learn the modes by going from C Ionian to D Dorian, etc. totally confuses what modes are. Each one is a separate SOUND and simply playing D E F G A B C doesn't tell you anything about Dorian.
However, if you play C Ionian (C D E F G A B C) and listen to it (sing each note as you play it, or better BEFORE you play it, you'll hear the sound of the major scale. Now, play C Dorian (C D Eb F G A Bb C). Same root, but different intervals. THAT'S what will get the sound Dorian in your head.
Why do I despise the "play D Dorian then G Mixolydian the C Ionian for Dmin7 G7 C" crap? Because if you really understand harmony you know that those three chords relate to each other in the key of C. They define the key-center of C and to think of them as three separate entities destroys the connection between them. If however you understand chord tones you'll already KNOW that your strong target tones are the D F A C then G B D F then C E G B, and your passing tones come primarily from the key of C. No reason at all to mentally or musically switch gears with each chord change.
Where to use modes? Well, let's look at a simple two-chord vamp repeating. Amin7 to D7. That's clearly NOT the key of G, even though those two chords in isolation in a song progression would define C. But for this two-chord vamp (like in Santana's "Evil Ways"), it never goes to the I chord. There's no melodic nor harmonic pull to the C tonic. Therefore I'd approach this as A Dorian. Not that I'd think of it as "the G scale starting on A", but rather that I'd be thinking of it as A B C D E F# G, thinking of the minor third between A and C, the perfect fifth between A and E, and the leading tone between F# and G.
But before you concern yourself with modes (and get bogged down in all the awful teaching out there on them, and before you get mesmerized by the "mystery" of modes or the "100 secrets of bass"), learn chords.
John