Well it might be useless for you to get acrobat...
but to us designers and people that require to open and edit documents all day long we only have Acrobat as a choice. We are used to it, we like it, it has other software does not have, etc.... Creating a document is not as simple as just creating it. With acrobat we have tons of options and tools that we NEED in order to get what we want or what's required, and you don't have those options in preview.
This is like comparing "Windows Picture and Fax viewer" with photoshop.