In the UK (JAR-land?) you can instruct on a PPL as long as you've demonstrated CPL level knowledge ie passed the CPL exams.
You can earn money as a pilot - including instructing &/or single pilot ops - as long as you hold at least a CPL.
You can work as pilot on multi-crew ops as long as you've demonstrated ATPL level knowledge ie passed the ATPL exams AND completed the MCC. I refuse to use the term 'passed' for a course that isn't assessed.
The beneficial part is that ATPL passes will credit you with CPL passes for no extra study. CPL passes only give you CPL passes ie no credit towards the ATPL.
The catch is that the ATPL exams only remain current for a limited time AFTER you gain a CPL licence + IR.
Given that, you may as well sit the ATPL exams if you have ANY intention of moving beyond instructing. If not then ATPl exams may not be relevent to you.