His proposal has much to recommend it.

问题描述:

His proposal has much to recommend it.
为什么要加it

这里可以有it,应该指某个具体的事物.例如,他的建议里书用了很多篇幅推荐使用某种新材料,这时我们就可以说:
His proposal has much to recommend using of the new material.
显然,可以用it来替代using of the new material.
不同意二楼的说法:His proposal就是主语.