I think this is a very good question, at least one that I have asked myself as I stare at 3 monitors every day. The reason I ask is because when I go out of town and still trade, I take only a laptop.
The reason I like multi-monitors is because I like to look at several time frames at the same time, without having to "flip" back and forth as you said. My pea brain does better with all of it at once than in bits. Even on the laptop I reduce 4 charts to fit the screen and have them set for the time frames I like.
Btw, I just bought the MAG 18" LCD from BestBuy today for 399 after rebate. I've had good luck with MAG crt's but my eyes are getting hard-boiled so I thought I would splurge a little. So the new MAG will take the center position and now I have an extra 19". LOL, would you like to double your viewing pleasure?