Interesting. When you say DIY you mean pencil and paper. For most of us the choice came down to using a professional vs using the software.
Your second bullet really hits the point. The tax return is a giant spreadsheet with multiple cells depending on each other. Short of building my own spreadsheet to perform the task, I found the software, at $30-$50, to be the happy medium between the full DIY and the Pro at $400+.
With a single W2, and no other items, the form is likely just a 1040-EZ, and there shouldn't be any recalculating so long as you have the data you need. Pencil/paper is fine.
There's no exact time to say go with the software, except, perhaps, when you realize there are enough fields to fill out where the recalculating might be cumbersome, or the need to see the exact tax bracket has value for you.
You are clearly in the category that can fill out the one form. At some point, you might have investment income (Schedule D) enough mortgage interest to itemize deductions (Schedule A) etc. You'll know when it's time to go the software route.
Keep in mind, there are free online choices from each of the tax software providers. Good for simple returns up to a certain level. Thanks to Phil for noting this in comments.
I'll offer an anecdote exemplifying the distinction between using the software as a tool vs having a high knowledge of taxes. I wrote an article The Phantom Tax Zone, in which I explained how the process of taxing Social Security benefits at a certain level created what I called a Phantom Tax Rate. I knew that $1000 more in income could cause $850 of the benefit to be taxed as well, but with a number of factors to consider, I wanted to create a chart to show the tax at each incremental $1000 of income added. Using the software, I simply added $1000, noted the tax due, and repeated. Doing this by hand would have taken a day, not 30 minutes.
For you, the anecdote may have no value, Social Security is too far off. For others, who in March are doing their return, the process may hold value. Many people are deciding whether to make their IRA deposit be pre-tax or the Post tax Roth IRA. The software can help them quickly see the effect of +/- $1000 in income and choose the mix that's ideal for them.