true, if you hold a Cdn/UK/US/AUS etc passport, then a HKSAR passport is unnecessary. However, if you don't hold one of these (easy-to-get-visas-with) passport, then having the OPTION/ABILITY to get a HKSAR passport is of GREAT importance!! Think of all the visa-free countries one can travel to with a SAR pass, not to mention the ease of traveling to China.
Basically, if you already hold a "first-world" passport/citizenship, it would be irrelevant. However, not everyone in HK (or born in HK) is from a first world country (by nationality) and traveling with these passports can be very frustrating. (i know, my husband holds a "visa-restrictive" passport and it's a b****h getting/applying for visas for EVERYWHERE - - - even tho he has his PR in HK. he's not eligible for a HKSAR pass b/c he's not ethnic chinese.