The answer to (1) is that there is no book for index spread orders on CME/CBOT. There is significant margin relief ~75% reduced margin.
The answer to (2) is that in my opinion it is worth trading them. However, it is not easy and requires you to understand how these markets work in relation to each other. Also, these spreads can be an interim position in between outright trades (CROSS HEDGE).
Look at today's chart of NQ/RTY spread.
View attachment 208458
This is an NQ/RTY spread with a weighting long 1 NQU9 and short 2 RTYU9. ES is in blue and the spread is in white. ES values on left and the cash value (notional) of the spread is on the right. Lower chart is detrended against the red line. The red line is a polynomial regression on the spread.
This spread had significant volatility during the open and subsequent rally in S&P 500.
This is a market cap spread between the top 100 non financial stocks and the largest of the small cap stocks. Strong buying or selling in the ES greatly affects the RTY contract (typically).
The way I would trade this.
(1) Put the spread on at the open.
(2) At the top of the rally in the spread I would check if ES is bullish or bearish. Today seems bullish at 9:55AM. In this case buy back the RTY contracts.
(3) So you are long NQ while market is rallying and also you made money on the spread rally!
I like to trade this way.