IMHO, your misconception is that one, and only one, mode of transportation is sufficient for everybody. Transportation infrastructure is much more complex than that, especially in a major metropolitan area such as your own.
Now if Houston was a small city of 25K people or less, I'd agree with you, light-rail would be totally ludicrous. But Houston is nowhere near that small, and a variety of means of transportation should be available to suit the various needs of different people.
If you put in hundreds of lines, you might serve the community. Maybe.
A variety of transportation systems is fine, but that doesn't mean we have to install every single type of transportation known to man just because it is there. Some types work better than others in Houston and some don't work at all.
With rail the fact of the matter is that it simply doesn't work in Houston. It is not economical. It is not efficient. It does not operate well. It does virtually nothing to combat traffic congestion, and in fact in many cases it makes it worse.
Unfortunately Houston has learned this the hard way and, if the current line gets developed, will learn this the hard way a second time over. We had a rail system for over 70 years that was one of the most extensive in the nation by 1927. It was not efficient, nobody liked it, and it sucked up huge deficits every year. The invention of cars and busses, and later freeways, spelled the end for rail in Houston. Rail did not go without a fight though - they used city government to try and make themselves a monopoly for decades past their prime. In 1924 they even got the city charter amended to ban taxi cabs because they competed better as a means of transit. It was gone by 1940 though because rail simply didn't work.