As scifan57 says, if you just want one app, then purchase it. Remember, the developer has spent time and money for developing the app. S/He deserves monetary benefit out of it. Apple will slice his cake by 30%, thats another story. I will not jailbreak my iPad just for one app (that I wish it was free).
I jailbroke my iPad yesterday because I have a 4 year old. I work on my iPad and my work is important. There is no guarantee that my toddler won't fire up my apps and will not mess around my work. I need more parental control on my iPad. These offered by Apple is a joke and since I am on iPad 1st Gen, apple won't give me any more updates. Hence, to have more parental control on my iPad, I jailbroke it.
BUT
I am yet to find an app on Cydia that satisfies my needs.
So in short, my for me, it didn't make much difference.