Nothing I hate more than a boat where all the lines are cut from the same spool. Trying to untie the inevitable knots is much easier when you have different color lines.
Secondly different uses require different lines. They aren't all the same, and despite some cruisers recomendations Sta-set is some of the most expensive line on the planet when sized for breaking strength. Selecting proper line is critical for a boat to work at its optimum, AND is less expensive than just buying off a spool.
Primary considerations for different uses
Halyards - low stretch, light weight
Traveler - hand, low water absorption
Main Sheet - low stretch, low water absorption
Jib sheet - able to be put on a winch, low stretch
Docking lines - nylon
For all lines, remember, if you are paying someone to splice anything but three strand line you can probably learn to splice it pretty easily.