The Left uses him to paint conservatives with a broad brush of "out there" craziness, largely drawing on soundbytes that probably do sound whacky to your average working Joe when presented, as I discuss below, almost necessarily out of context.
The people who want desperately to be known on the left as the "reasonable right", then, uses Beck as someone on their side to distance themselves from the painting the Left has garishly painted with their 12" paintbrush.
If the "reasonable" right actually paid attention to Mr. Beck over a period of time, they'd realize he's pointing out some things they ought to be looking at seriously. But they don't, because it would take too much time and they'd be afraid of the Big Brush™ Off.
As an example, Fareed Zakeria, a good guy who writes some good articles wrote the other day about Egypt's "real" (his word) paralell to Iran's revolution sort of defends Glenn to some extent but still can't help doing it using the back of his hand.
"Fears of an Islamic takeover are not limited to Glenn Beck, with his predictions that the fall of Hosni Mubarak will lead to the rise of an Islamic caliphate bent on global domination. (Beck's policy recommendation to Americans was even more out there: "store food.") Serious conservative politicians such as Mitt Romney and John McCain...After you've picked yourself up off the floor laughing about Romney and McCain being "serious" conservatives, recognize that he's not so much calling them "serious" as he is dismissing Glenn Beck as not being serious. Glenn is dead serious. But you have to have an attention span longer than a 6 paragraph editorial to gather the whole picture as to where Beck is coming from.
Of course, the above is a gross overstatement of what Beck is saying. But Fareed probably doesn't know that, nor is he interested in finding out. Glenn's too convenient to use as an anti-yardstick for "serious" conservatives.
What Glenn has said is that that is the intent of groups like The Muslim Brotherhood, and increasing their power in Egypt is a step in that direction. (update: apparently Don Rumsfeld, Diane Feinstein, & General Abizaid agree with Glenn. And of course, we already knew Al Zawahiri agreed. HT: StuBlog) There are lots of other groups in the middle east (and elsewhere) that share this goal, and we should pay attention to them.
Beck's "store food" meme goes more toward the possible (and "prediction" probably isn't an overstatement for his position on this one) that trying to print our way to prosperity might lead to a Weimarian bout of hyperinflation. It has nothing to do with Egypt or a possible Caliphate.
But it's not just crazy paranoia here, either. I don't hear people calling out Warren Buffett for suggesting every American should read "When Money Dies". He's not exactly considered a kook in most parts.
You can't just pop in on Beck now and then and get an idea of what he's talking about, because it's part of a much longer, broad conversation he's having with his audience that can't be reduced to soundbytes that are bound to be presented out of context.