Yeah, you could do that as well. I just don't like the idea that someone holds my email address. After reading few stories where Google/Microsoft blocks access to email, I decided to move my email to custom domain. In case of any issues all I have to do is change MX names to new provider to start receiving my mails again.
Yes, but one of the great things about having GApps with your own domain name, is that you are not tied to Google as an email provider, and can easily switch. If you use something like the '+' modifier, then if you switch away you need to switch to someone that supports the same modifier.
So if your goal is to maintain email-provider independence, then relying on provider-specific features like the '+' modifier works against that goal.
For me though, the main reason I don't use it is most sites that I want to use it on reject the '+' in the address as invalid. It happens enough that I don't bother trying anymore.