In a perfect world, you wouldn't need an adjustable bridge. Strings would vibrate perfectly, regardless of their gauge or construction. One end of the string would be fixed at the nut and the bridge would be a straight line parallel to the nut one scale length away.
Of course, real strings aren't perfect. They need quite a bit of mass to be tuned down this low and there are infinite combinations of core wire diameter, wrap wire diameter, number of wraps, and materials that all need to come together to make this happen. What you end up with is a string that has stiffness, generally more stiffness as you get to the bigger strings. The stiff end of the string won't vibrate as easily as the middle of the string, so it acts as if it were shorter than it really is. Unfortunately, each harmonic sees the stiffness differently, so a fundamental and it's harmonics will tend to be slightly out of tune with each other.
The intonation adjustments you do at the bridge try to compensate for the string's characteristics. On most basses, you'll see that you need to pull the bridge saddle back for the lower strings. The extra length compensates for the stiffness and hopefully will bring the note and harmonics back into tune with each other.
The proper intonation point is different for each string, but (assuming the manfacturer holds tight tolerances) you'll probably find that that point doesn't move over the life of the string or when you put on a new set of the same gauge. Change material, gauge, manufacturer or even your playing action and the proper setup will probably be different.
If you settle on one string setup, then you don't need adjustability on your bridge - you just need the saddles to be at those points that you determined to be the right ones. The problem is that people do change, and even if you don't, there's no guarantee that the strings that you like will place the saddles in a straight line.
Acoustic instrument don't have individually adjustable bridges like an electric guitar. That's because the bridge is primarily responsible for carrying the sound of the instrument to the soundboard. For an acoustic guitar or even an upright bass, you want that bridge to be light and totally solid. There, you trade off perfect intonation for better sound. On an electric, with magnetic pickups, the adjustable bridge gives you better control of intonation and playing action.
If you look at an Ovation roundback acoustic, you'll see that they shape the saddle to try to help the intonation a bit. There are similar steps on the old pre-Tuno-o-Matic bridges on Gibson electrics in the early 60's. Older Martins had split bridges too. Because you're more likely to be playing chords on a guitar, I think intonation problems are more noticeable on guitars than basses, but the problem with basses are that there needs to be a lot of bridge adjustment to fix the problem, so you're much more likely to have huge intonation errors.
The bridge on an upright bass is held down to the soundboard by string pressure alone (there is a post underneath the bridge to counteract the downforce), so it can be adjusted for particular strings, similar to what 88persuader mentions. I think you'll have a hard time generating enough downforce across the bridge for this to work well on an electric bass though. On an upright, the string end past the bridge is inches long and the anchor point is inches lower than the bridge, so there's a lot of force to hold the bridge in place. On an electric, the body is a lot smaller, so you'll have to do something to find an anchor point that will give you downforce. This is what that tail on Les Claypool's Carl Thompson bass is doing (it also has a non-adjustable bridge).
Of course, you'll have to machine your bridge to give you the action heights that you want as well, and those too change with strings. On one hand it's a pain in the ass to do all this stuff, on the other, this is sort of what acoustic guitar players have to deal with all the time. They have no choice, however!
If you're building one bass, then I think the non-adjustable bridge is going to be a lot of work. For a luthier who can incrementally try a lot of strings and locations, there's a much better chance that they can get good results with intonation and action adjustments.
One thing that you might think about trying if you want things to be different is something like the bridge used on Spectors and Steinbergers (on the headless Steinberger, the knob tuners are in the bridge too, but this is separate). The main body of the bridge is a big tray. Each string saddle is on a beefy rectangular block. Each block has set screws perpendicular to the bass body that adjust for the height and angle of this bridge piece. You string it up and scoot the blocks back and forth until the intonation is right, then you tigthen a setscrew at the side of the bridge which squeezes all the bridge pieces together, side to side. Once you've done that, the entire assembly is locked together as one and doesn't depend on screws or springs to hold it's position, so you get the benefits of solid construction without losing any adjustability. Way cool.
David Fung
