I'll give you an overly simplified example, so we'll take it with that caveat.
When you invest in an RRSP in Canada, you might ask what your purpose is of investing in an RRSP. Is it to save for your retirement, or is it to avoid tax by getting a deduction on your RRSP contribution?
If you ask 10 different individuals, they might give 10 different answers. Some might say they did it just for retirement. Some might say they did it because they wanted the deduction. Many would say they did it for both reasons.
What this principal purposes test would do, if it applied to an RRSP deduction, is allow the government to step in and determine whether it thought the tax savings was one of your principal purposes, and if it were, then it would deny that deduction.
But I just use that as an example where it's difficult to determine at what point a purpose becomes a principal purpose, and whether it's one of your principal purposes. Quite frankly, in making any investment in any country around the world, if you don't take into account the tax result, you're negligent. It's part of computing your after-tax returns.
Because tax is always considered, it's very difficult to determine at what point you just considered the tax results and at what point it became your principal purpose.
And finally, on my RRSP example, I would say that it shouldn't matter in that example whether one person's purpose was retirement savings and another person's purpose was to get the current deduction. We should treat them equally, regardless of what their purposes were.
That's my other concern with the test is that it doesn't necessarily treat equal taxpayers equally because it bases the result on what their purpose was, as opposed to what they objectively did.