1. You're using latest upstream, after it passed through the archlinux-testing repository. This means you won't have to install software from somewhere else just because the repos are outdated. (nighly builds of software are different)
2. Sometimes you have to sys-upgrade to install/fix something, and you don't have much say in when that happens. Typically you need to preemptively do this about once a week to not potentially get interrupted by manual intervention at a bad time. Yes, you will need to do manual intervention, but you won't spend much time on it. It's far less work than getting up-to-date software running on your debian stable.