When we last went to WDW, my children were 2, 4, and 6. My 2 year old definitely needed it and I thought my 4 year old would too. It ended up my 6 and 2 year old used it the most. My 4 year old was very energetic and simply just wanted to walk. My 6 year old had some virus the whole week that caused a headache and 101 degree fever (so did 2 year old). Luckily Tylenol took that all away, so they lived on it all week! But anyway, due to his illness that week, he was happy to have somewhere to ride. That and the fact that even if he wasn't ill, he's just the type of kid who'd rather ride than walk.
So it really depends on the child. If I were you, I'd take one or rent one just in case.