I think that learning to play legato is hugely important in one's development as a bass player. Just like learning to play loud, or soft, or slap, or finger, or pick. It is one of the skills that you are expected to have and it comes up all the time. I think there are two elements to it, a right hand and left hand. Learning to shift position smoothly and quickly is essential. I'll give you a little left hand exercise that my teacher gave me, that transformed my playing (this was very early on in my studies )
1. finger the A on the G string (2nd fret) with your index finger. Stike the note, and then slide into the 3rd fret, strick the note and slide back to the 2nd fret. Strike the note again and slide into the 4 fret and back. Repeat sliding into the 5th, 6th, 7th, and so forth until you are sliding the octave (14th fret)
2. Here is the challenge: you must be able to do this so you can't hear the slide!!!! You must fret the note, lift pressure just a little, make your slide, then fret the note you are aiming for. Easy to do on the first couple of frets, hard on the octave (or above) It should sound like you are almost hammering the next note. The goal is to have smooth, legato phrasing, playing huge position shifts, with the same finger. You must not be gripping the neck too hard, but have a light touch with the left hand. Thumb must be properly behind the index finger (c shaped grip) REPEAT using ALL 4 FINGERS!!!!
3. This exercise will really increase the accuracy and speed of position changes, which is maybe why some have trouble playing smoothly and with legato phrasing.