I think parents like to do surprises in hopes of seeing a reaction of pure joy and delight in their children. If this is what you are looking for and think a surprise is something you're kids will love, by all means go for it. I know some parents who have done this, and it's a super fond memory for both parents and children that lasts a lifetime. Not everyone in the world, however, likes surprises though, so you need to be prepared for that.
Now me, I've never considered surprising my son, but so much of that is due to how much fun I have doing the vacation planning with him. It's just such a great activity to do together. Also, I'm also a person who personally doesn't like last minute surprises, so planning one myself would be a big paradigm shift for me.
Here's sort of what goes on in my head with the kind of surprise you are talking about: If I was anticipating something else and all of a sudden this is what we are doing, it's a "curve ball" for me that hits me in the face. And instead of being excited going some place I really love, I think about how that's messing up the other plans I have and feel sort of bummed that I'm not going to be doing the other things I was looking forward too. And then on top of all that I feel bad about my reaction as it's a disappointment to the people who are surprising me who I care about deeply and had such good intensions. Anyone else ever feel like that? -- include me in the planning, give me some say, and it's so much better for me and I'm so much happier.