In the New Scientist, Harvard theoretical physicist Lisa Randall was asked about the concept of a “theory of everything.” The title of the article sums up her response: “A theory of everything won’t provide all the answers.” (Emphasis added.)
Randall doesn’t think the pursuit in itself is foolish, but she explains right off the bat that it’s presumptuous to expect all physicists to care about it. And even if the “idea that we will ever get there is a little bit challenging,” a theory of everything would leave some of the most important questions unanswered. Asked if it’s every physicist’s dream, she says:

There are lots of physicists! I don’t think about a theory of everything when I do my research. And even if we knew the ultimate underlying theory, how are you going to explain the fact that we’re sitting here? Solving string theory won’t tell us how humanity was born.

Randall’s answer points out the fallacy of the reductionist project in science. Would a theory of everything really be useful? Surely the most important things to us as human beings would remain unexplained.
For instance, suppose someone like Randall answered every question from her husband or teenage daughter with, “Because the big bang happened.” The husband asks, “Why were my car keys left in the car?” “Because the big bang happened.” The daughter asks, “Why don’t the boys at school ever ask me out?” “Because the big bang happened.” A neighbor asks in tears, “I don’t understand why, no matter how much I do for them, my kids don’t respect me.” Our scientist responds, “It’s easy to understand. It’s part of our theory of everything: the big bang happened.”
An overarching goal by some scientists has been to explain particulars in terms of universal laws: to reduce lower-level explanations to higher-order principles. The basic idea is: biology reduces to molecular biology; molecular biology reduces to physics; physics reduces to particle physics, and particle physics reduces to the big bang. But it’s not clear that more isn’t being lost than gained by this reductionist project.
Often in science, the pursuit of understanding focuses on a far more particular level than universal principles. The ideal gas law may constrain a question about the role of nitric oxide in cell signaling, but it is only peripheral to the main question about NO’s function. The law of gravitation affects all bodies, but is so negligible in terms of cellular processes as to be useless. The laws of thermodynamics (some of the most universal laws in all of science) will probably not even be mentioned in a paper about the classification of a new insect.
In his lecture outline for “Philosophy of Science,” produced by the Teaching Company, Jeffrey Kasser points out a number of problems with the reductionist project, not the least of which is the tension it creates between the metaphysical aspirations of science and the requirement to ground scientific explanation in empirical observations:

In general, the direction of reduction is away from that which is epistemically accessible. Relatively observationally accessible notions, such as temperature, are reduced to relatively inaccessible notions, such as molecular motion. Hence, we see recurring tension between the epistemic modesty emphasized by empiricists and the explanatory, reductive, and metaphysical ambitions that seem to crop up as we look at science’s aspirations.

Advocates of intelligent design would argue that a “theory of everything” that leaves out intelligent causation in explaining the history of life is doomed to failure. You cannot reduce the genetic code to hydrogen bonds or laws of mass action any more than you can reduce poetry to the big bang. Since intelligence is epistemically accessible through our uniform experiences of intelligent causes, ID can ameliorate some of the tension in science between epistemic modesty and metaphysical ambition.
ID advocates are careful, unlike materialists with their theories, not to slip into the fallacy of calling ID a “theory of everything,” to which everything reduces. ID explains some things well (e.g., specified complexity) but leaves many other phenomena to natural explanations. The quest for a scientific theory of everything is in fact a fool’s errand.