When I select a rod for the day (or the run) I try to factor in the following factors:
1) The fly and line I will be using
2) The water I am fishing and when I am fishing it
3) The fish I am chasing
I list them in that order because I feel that is the order of importance. The answers to those questions will determine if I fish a 10, a 9, an 8 or maybe even a 7.
First off, if I am fishing heavy tips and big bushy flies or heavy tubes, I am limited in rod choice. A seven or an eight won't allow me to cast and fish like I would like. A good 9 weight will handle quite a bit but when the tips get too big (heavy and/or long), it becomes a chore to cast with a nine. Granted, a nine can be made to work especially if you are fishing a shorter bellied Skagit style line but even then work is the key term. A ten will easily handle these combinations.
Rod choice also is impacted by the water you are fishing. If you are fishing in December and January, fish are less likely to come up in the water column for an offering than in March and April. Add to this low, clear and cold conditions and you end up with many fish hunkered down in deep pools. To get to these fish you need to get down (see #1 above). Rivers on the edge of fishable due to freshet also sometiumes call for a 10. While it is true that fish will often hold on the shore in these conditions, it is also true that they will hold near structure and on seams in faster water than you would normally fish. Once again, the 10 comes in handy. It is not uncommon for me to fish a 10 quite a bit in June for this reason. Finally, river size is sometimes a factor. While I would not use a 10 on the Snake in most circumstances, I am more likely to go to a 10 on the Skagit than I am on the N. Stilly.
Which brings us to the fish. If the fish was the only factor, I would tend in most cases to agree with you that a ten can be overkill. Most cases but not all. I am sure the Thompson boys will chime in here. There are just some rivers where the fish should not be hooked on anything less than a solid 9 weight. The Thompson and Kispiox come to mind but even the Sauk, Skagit and occasionally even the Sky have fish that fit the bill. And then there is the health of the fish to worry about. While I can land a high teens fish on a seven weight, it is much quicker and far easier on the fish to do so on a 9 or 10. This becomes critical for summer run fish in higher water temps.
The last point I would bring up is that line designations for speys should not be equated with the same uses for single handed rods. I personally would never use a 10 weight single handed rod for steelhead and in fact, in most cases, think a 9 is overkill. A 10 spey has its uses for steelhead though.