Wrong. That depends on the licence you use. For example the GPL states that the source and project are free and MUST REMAIN SO. You can charge for addons tho. You can make a commercial license saying... Sources are free AS LONG as you don't use them in a commercial application. If you want to use them contact us and we can come to a deal.

It really depends only on the way you formulate the license.

The advantages to opensource are that you can get free bugfixes from the comunity and in the end the product will be much better. The disadvantages are the danger of thieves(they will take it and use it to make moeny) but you can always have the fun of sueing them

Anyhow one BIG disadvanteg(for us) of you not going OS is that we'll lose the project for good if you die(I mean software wise).