The big rods are used nearly exclesively for winter fish and large flies. I use my 16 foot T&T only from November to May for winter fish unless I go up to the Thompson and then it goes up there with me as well.
The rod I use the rest of the time is a fast 8/9 weight 2-hander in either 13 foot or 15 foot. And that depends on whether I am planning on casting 80+ feet or not. If it is going to be an over 80 foot day, I use the 15 footer, uner 80 feet, I use the 13 footer.
Avoid the slower rods because they will allow you to develop poor spey casting habits. Likewise, use one the mid length belly lines (MidSpey, Long Delta, etc.) that are 65 to 70 foot belly lines. They help you develop better casting technique from the beginning and are far nicer fishing tools than the short belly lines (Windcutter for instance) of 55 or less foot belly.
Why use a 2-hander when you already own an 8 weight single-hand rod? simply, command of the water and ease of casting when the brush is next to you. Do you need to have one to effectively fish for steelhead, nope. Does one help you to fish better and more effortlessly, without question.