1. You have to sign in to an Apple ID to download Sierra, it doesn't matter which you use.
2. The Mid-2009 MacBook Pro works with Sierra 100%, I own one and have verified this.
3. There's no security issues as far as I can tell.
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The tool is updated and should work fine with the final release.
dosdude1, thank you for your quick reply.
I want to ask a question about Question 1 in detail.
I will purchase(download) macOS Sierra with my friend's MBP.
My question is that I should log-in with my Apple ID to purchase Sierra due to later updates?