The one thing your overlooking here is soil itself is not a buffer, it's certain organic and inorganic elements (like lime) that exist in most potting mixes that help "soil" buffer pH. These elements break down or wash out of the medium over time and become less effective. For many standard grows this isn't an issue, but if your vegging for a really long time already in the final pot or growing in really small pots sometimes these buffers will be ineffective before you finish your grow. Also, it would appear salt build up from bottled nutes can affect these buffering agents ability to do their job. I found this out the hard way in my last SOG grow. I wound up with a really low root zone pH. The only way I could keep an appropriate soil pH and continue to feed my plants was to raise the alkalinity of my nutrient solution. So there's a first hand example of soil not buffering alkalinity.
So do all soil growers need an expensive pH meter? No, probably not. But if you end up with pH issues like I did, you will, and it's not like it hurts to have one.