While, IMHO it's not necessary to have an itinery that's cast in stone and that you have to follow religiously, I do think that the majority of people who take the time to draw up a plan of attack find it useful.
My own way of doing it was to have a rough guide to stick to and if circumstances came up to change the plan, then it was easy to juggle things around and swap other day's activities into that particular day. My plans would go something like this
day 1) arrive, check in, grab a pizza go to sleep
day 2) Animal Kingdom for the day, leave at 4.00 go to Epcot eat at Le Cellier watch fireworks
day 3) brekkie at Parkfare 1900 (GF hotel) day at MK
day 4) Typhoon lagoon evening meal at Boma's
day 5) shopping at florida mall, eat at Bucca Di beppi's (in florida mall)
day 6) IOA
day 7) 2 hours shopping at Belz, rest of day at SeaWorld
day 8) Blizzard beach
day 9) Seaworld up to 4.00 , then shopping at Belz for a couple of hours, go to Mall at the Millenium for a couple of hours, eat at cheesecake factory
Day 10) Disney Quest and shopping at DTD
Now if it's predicting it's going to be sending buckets down with rain on days 4+5 I'd switch day 10 (DQ) with the water park day and juggle the rest of the days so everything gets fitted in.
I've found the alternative, no plan, tends to mean that if you get tired (too many themepark days in a row) or the weather throws you a curve ball, it's easy to get to the end of the holiday, have 8 days worth of themepark tickets bought and paid for and only 3 days to use them in. An itinerary, even a rough one, before you go will help highlight the types and amount of themepark days you're going to need. IMHO the biggest single financial mistake people make is over estimating their themepark ticket needs and wasting money on park days they never use. I'd guess I wasted about $700 on my first visit because I deciced I wanted to "do everything" but didn't think about the logistics of actually getting to use all those themepark days. With a little planning I could have put that $700 to much better use.