It can only be a bonus mate, and it's something I am going to be doing myself sometime this year.
Any good 2 week plumbing course will give you what you need to fit bathrooms and any general wet plumbing that you may require, and can only earn you money from then on being able to not have to call out plumbers yourself.
2 bites of the cherry mate
Just remember to add plumbing to your liability insurance.