If what you want to do is leave your 13m home, then do so without guilt. It is an individual family decision. I don't think parents are awful for doing so, but I would never take one (or more) children with DH & me and leave one (or more) behind. For us, THAT is a family vacation. And, to me (I may be wrong), if doesn't sound as if you WANT to leave your 13m at home.
We took DS#2 for the first time at 7 months (DS#1 was 3.5). It wasn't easy per say do to all his extra health issues. But it was worth every second. Yes, a baby can enjoy things, and learn and benefit from them without remembering them. I did not find WDW any more difficult than any other day at home. Then there was the extra benefits/memories of things that would not have happened without him there. Pocohantis got down on the floor and played with him, and him alone, for over 20 minutes. There were many other experiences like that which are totally priceless and don't happen with children who are up and walking. I have many adorable memories, and pictures, that I wouldn't trade for anything in the world.
We're going back in May and DS#2 will be 15 months (DS#1 is 4 & I'll be 5.5 months pregnant). He only slightly tolerates the stroller, doesn't like to be held, won't hold anyone's hand, and is just down right irritable (sp?) most of the time. I think it would be the most miserable vacation without him. Being without him for 11 days would kill me. Not to mention knowing all that he would be enjoying while we were there. All those pictures without him... And I know he can remeber it, he remebers other things all too well and he is hardly an advanced child.
We have another trip planned for December 2006. DS#1 will be 5.5, DS#2 will be one month shy of 3, DS#3 will be 15 months, and if all things work out I should be about 5 months pregnant with #4 (unless we do decide 3 is enough). Both dh & I are looking forward to this trip.
You may also want to consider this: The first time my parents took my sisters to WDW, they left me behind with my grandmother. It was such a huge regret on both my parents part. They went and got me, and took me with them the next day. According to my mom, they all had a better time with me there. I was 2 and my sisters were 9 and 7.
If I hadn't gotten pregnant, I was going to take DS#1 to WDW shortly before his 4th bday as a special mom & son treat and not take DS#2 with us, but DH wasn't going with us either. Unfortunately, I wasn't going to be able to take DS#1 on 1/2 of what he loved best (Goofy's Barnstormer, Dinosaur) being pregnant. The first trip we baby swapped, this trip I can't go anyhow so we'll just sit out the height restricted rides.