Ok, you guys are confusing the idea of "/cast [target=self] Chain Heal" with actually targeting YOURSELF. Try it. Your target does not change. The macro points the cast at a particular target, but it does not actually change your target. That would require a "/target myownshaman;/cast Chain Heal" to actually change my target. I tested his macro. It is not spammable in any form or fashion.

Edit: I just saw your question in my code. You have 2 choices: you can either /targetlasttarget at the end to "ignore" the targeting of yourself earlier in the macro or you can add "/target [combat] thisshaman" instead of just the "/target thisshaman". Take your pick.