I can't speak too directly to whether The Economist is a great fit for practicing LSAT RC.
Unfortunately, I'm not much of a reader, except for the occasional nonfiction book that attempts to make quantum mechanics and theoretical physics understandable to science-morons like me.
I will speak more broadly to the advice that LSAT students (or anyone struggling with RC) should get in the habit of reading more challenging reading material.
The virtues of this plan ---
1. You'll hopefully teach yourself to take an interest in potentially boring, obscure topics.
2. You'll hopefully become more familiar with some concepts/vernacular outside of your current comfort zone.
3. You'll hopefully get practice trying to simplify complex written ideas into your own, simpler mental understanding of them
The question marks about this plan ---
1. Omigod, you're not practicing LSAT RC? Why aren't you practicing LSAT RC? Can you afford to be spending time not practicing LSAT RC?
I'm, of course, sensationalizing the problem, but it gets to the heart of my hesitation with this plan. The best way to get better at LSAT RC is obviously going to involve practicing LSAT RC, for a couple reasons:
1. They specifically pick and edit passages to have a recurring feel / difficulty level. The more LSAT passages you read, the more new ones start to remind you of old ones. And well they should -- after all, the LSAT is a "standardized test". A lot of what makes RC easier for me now than when I began is my awareness of common recurring purposes in LSAT passages, purposes such as:
-- Clarify a Misconception
-- Defend against Criticism
-- Highlight a Deficiency (and prescribe a remedy?)
-- Discuss How New Research affects Old Thinking/Problem/Paradox (most science passage)
-- Informative Discussion with some Noteworthy Interest
-- Present a Debate (and take a side?)
Most passages I read fall into one (or more) of those categories. I would worry that if I were reading non-LSAT material, I wouldn't be getting the same reinforcement when it comes to recurring passage patterns.
2. Half of Reading Comp is answering questions. A lot of people think they're understanding the passage just fine but still struggle with the answer choices since many of the questions have a lot of nuance/confusion/trickiness to them. You won't be honing your skills at this part of RC if you're just reading other dense material.
In the end, I think reading something like The Economist is great if you only have 10-15 minutes before you go to bed, or on the subway, etc.
But I'd rather have you do every LSAT RC passage twice than to use auxiliary resources to occupy primary study time.
I don't know if you've seen this, but in glancing at The Economist's website, this archive seemed like the best way to get some good, argumentative LSAT reading material from them:
http://www.economist.com/debate/archiveHave fun!