If there wasn't any proprietary applications or Operating Systems and all software had to be open source would we be better or worse off?

Advantages;

Viruses, viral marketing spam wouldn't be as bad as it is now.

Without vendor lock-in anti-competitive practices hardware compatibility problems wouldn't be as bad either.

Software would be selected on it's merits not on the size of the marketing budget, legal budget, dirty tricks budget...

Disadvantages;

There might be some but can't think of any right now.