We were at Disney last March 20th for Spring Break. It's true that the weather vary dramatically. Daytime was pleasant/warm (shorts, short sleeved shirts) but not hot, evenings were cool (jeans, long sleeved shirt/light sweater). HOWEVER, there was one evening when it was COLD...we wore jeans, several layers on top, and still had to buy sweatshirts for the girls (6 and 5 year olds) to wear. The Disney stores had to bring out the sweatshirts that were already boxed to go back to the warehouse. There were two ladies (Moms!) who got into a fight over a sweatshirt.
That night, we had the park to ourselves (seemed like) because when we came back to MK at 7PM, droves of people left the park because they were too cold (they wore short shorts and halter tops and couldn't leave the park fast enough). We rode a lot of rides over and over because the park didn't close until 11:00.
The people monitoring the monorail wore long coats and gloves. Of course, they are not used to cooler weather. I guess it depends on how cold it get where you're from. I'm in central Alabama, and it can be both hot (90+ degrees) and cold (23 degrees). In general, I'm cold when the temp is 60 degrees. We're going back in March and I plan to bring plenty of clothes options! Hope this helps.