Well...there are two schools of thought on this...hear is mine.
I use a fixed spool and 15 pound braid for light lures such as original rapalas, husky jerks, small spoons, spinners and some rubber jigs. This is coupled with a light but strong spinning rod of about 7-8 feet rated for about 5-20g. I fish for pike and Perch using this.
I use a multiplier when trolling, fishing large lures and spinnerbaitsor when jerkbaiting/using big rubber jigs. I have them loaded with 40 pound braid for normal work and 60/80 for jerkbaits. These are used on rods rated to throw anything from 3/8th of an ounce, right up to 5 oz.
So, to sum up, fixed spools are more frinedly to the lighter type of fishing, whilst multipliers are better suited to the medium/heavy side of out sport.
Hope this helps.