You may disagree with the logical coherency of his goals - I for one think the "temple for God" thing is pure kookery - but he is a master craftsman, and in this context it's all that really matters. There are people who devote their entire lives to pointless things. There are people who work on supposedly pointless things, which later turn out to not be so pointless. There are people who are supposedly working on world-changing things, which really are completely pointless. In the end, you can't predict the future. Just do honest work that you feel is worthwhile, and see what happens.
(if you want to play the hypothetical game, who's to say that Terry Davis won't develop some extremely efficient low level algorithms/techniques never done before that will dramatically impact computing?)
Every time his story comes up, there are people who respond with answers along the lines of "what can we do for him? can we find him assisted employment? can we raise money for him?" etc. While these comments are surely well intentioned, they end up being mostly condescending and out of place, as if talking about a completely helpless being. But Terry strikes me as anything but helpless - based on the various interviews he's given and comments he's posted, he seems to be quite content with his situation, and that "assisted employment" is the last thing he'd want or need. If anything, this is just a further argument for reforms like basic income and better treatment of mental conditions (e.g. with better early detection).
There are also always comments expressing surprise at how one can suffer from such a mental condition and yet do complex intellectual work like low level programming. These questions are based on the premise that schizophrenia (or a similar condition) impacts your brain in such a way that would make logical reasoning impossible. But that's not how it operates - logical reasoning is largely unaffected. Like `daveloyall says in another comment in this thread [0], what's affected are a few key "first principles". For instance, if you're persuaded that you're being constantly tracked by the CIA, removing all your clothes or dismantling your car to make sure you aren't bugged are very logical, reasonable things to do. The problem is that when you're operating under premises that are shared with no one else, the resulting dissonance makes integration with the rest of society problematic. But with this in mind, it's easy to see why one could be schizophrenic and yet still be able to program or do math or weave baskets, especially if the skills were mastered before the condition developed.
Terry is a maker of miniatures [1], and I'm happy we have such people walking among us.
[0]: https://news.ycombinator.com/item?id=8658958
[1]: http://www.newyorker.com/magazine/2006/04/10/in-the-reign-of...