I used a similar set up for a while (using two non-wide 20" screens). At first I thought of the vertical one my secondary screen, but then actually started using it for most of the real work.

It's great for reading web pages and PDFs and for programming, word processing and spreadsheets.

Didn't try using it for design or photo work, though.

I really hope Apple will rediscover the value of vertical screens one day.