On top of the informative post from RTN11, if you are not interested in flying Commercial Air Transport but nevertheless think that at some point in the future you may wish to add a specific Type Rating to your CPL/IR, e.g. King Air or Citation, you WILL need ATPL theory to undertake your first Type Rating.
The general advice is that, if you are ever thinking of going above and beyond GA flying for fun you should do ATPL theory. Once you have it completed, you have 3 years to get your CPL and IR issued and once issued the ATPL theory credits remain valid for 7 years from the date of your last IR proficiency check.
