In my experience, a standard setup on a four string allows you to drop your E string to D with little problem. Any lower than that, and you are going to run into trouble with the strings flopping against the frets, loss of note definition, ect. Therefore, if you play any lower than D on a standard four string, you are probably going to need to gauge up on your strings and get it specifically setup for that type of tuning. That's fine if you use that tuning all night. However, if you cover multiple tunings, you may need multiple basses. A five string eliminates that problem, since the B string is meant to be used for notes below standard 440. Throughout a gig, my guitarist uses four different tunings, standard 440, drop D, drop C#, and standard a whole step down. I can use the same bass throughout all of these tunings without having to switch instruments in the middle of a set, tune in the middle of a set, or maintain fresh sets of strings on a large collection of basses. Plus, being a bass player, I think in 4ths. I guess that "drop" tunings wouldn't be that big of a deal when doing covers if you're not improvising - I still love my 5 strings when playing cover material - but I definitely like having my bass setup in standard for the writing aspect of music. I do agree that a well-placed lower note can add a tasteful nuance to music. We cover Bad Day by Fuel. Anyone familiar with that song knows that it goes from D maj to D min. I play the D maj on the 5th fret of the A string, but then hit the low D on the D min. I believe that's how it is on the original recording. It really adds some tasteful power to the song.
As far as 5th fret E vs. open E - To my ears, the 5th fret B has more fundamental and more power, but the open E has more definition and upper harmonics. What I play has to do with the situation. Sometimes an open E makes more sense when considering the economy of motion. Sometimes a fretted E is better (especially since you aren't left with the issue of muting it after you play the note you need).
As far as 5th fret E vs. open E - To my ears, the 5th fret B has more fundamental and more power, but the open E has more definition and upper harmonics. What I play has to do with the situation. Sometimes an open E makes more sense when considering the economy of motion. Sometimes a fretted E is better (especially since you aren't left with the issue of muting it after you play the note you need).
