So, getting back to the original question [“Are tread patterns mostly for show?”], Tire Rack lists over 80 different all-season tires for a 2007 Civic, each with a unique tread pattern. So apparently the answer is YES.
Getting back to all your stipulations…“summer, bare dry pavement” , no snow, no ice, no wet weather, no gravel surface, where even a bald tire might be adaquate…then yes, they all ALL FOR SHOW… 
@ok4450 please read my comments again. The load index is just a number that says little about the effect of the load on handling. It indicates the load the tire is rated to carry. Of a traction tire is used at near it’s load carrying capacity and as is often the case, has a reduced speed rating along with poorer handing, that means we were better off using a traction tire with a higher load rating. We found this to be true with the vehicles we used for work. In this case, the tread design did have an influence on the load carrying capacity of the tires we used.
In this case, the tread design did have an influence on the load carrying capacity of the tires we used.
I would reverse that logic. The load carrying capacity (including all related performance requirements for that class of tire) requirements drove the design of the tread pattern.
There are a limited number of fundamental tread designs for various applications/conditions. Then everyone has slight variations to avoid patent infringments and perhaps provide some margin of improvement (real or perceived) over the competitive designs…
This idea they are strictly for show is goofy. It implies some designer working in a vacuum. Certainly, some truck tires are bought because the tread pattern is more aggressive looking but for a car? Frankly, I have never given it a second thought. I buy based on advertised and tested performance criteria…not because I liked the look of one pattern over another…
@twinturbo.
I think so too. i am not naive enough to think that given two tread designs that worked, the manufacturer might pick out the prettiest one. But, saying that is the primary reason for their difference is a real stretch for me to wrap my tiny little mind around. That doesn’t mean someone much brighter couldn’t see it differently. I just remain unconvinced it goes anywhere near that far and agree.
I think part of the issues being expressed here is the difference between 1) what it is that drives the tire design and 2) those things that are downstream of the process.
For example, 4 doors on a car is common for the family sedan, but sports cars have 2 doors. To say that anything with 2 doors is a sport car (or a sporty car) is obviously wrong.
The same is true of tread patterns. It’s the usage that is driving the tread pattern and the load carrying capacity. But the load carrying capacity is NOT driving the tread pattern, nor is the tread pattern driving the load carrying capacity. They are both the result.
Oh and there are patent infringement lawsuits on tire tread patterns all the time. Each tire manufacturer MUST have a different pattern. The good news is that there is more than one way to cook a goose.