Well, if you use this strategy you are basically artificially inflating the difficulty in your practice sessions. So the risk is that by making an already extraordinarily-difficult test even MORE difficult some students are either going to become discouraged or start using time-saving "tricks" that only cause them to make mistakes by being sloppy or not thorough enough or by skipping over important information while reading to compensate.
In addition, this strategy probably only offers real benefits to students are are shooting for an elite score, and know they can attain it. If you aren't able to fully read through and finish a section in 30 minutes, this strategy probably isn't going to help your confidence or abilities in any worthwhile way. So I actually agree with the prep books that the average LSAT-taker is probably better served by concentrating on getting more questions right, rather than focusing on speed.
That said, as you allude to another benefit of doing this if you ARE able is that it usually means you won't suffer a big "test-day drop" in your score since the practice tests you are taking are "harder" than the real thing. My real score matched my best practice score, and I know studying this way was part of that.
A good analogy might be practicing 35-mile runs to prepare yourself for a marathon. If you are just looking to finish, it doesn’t make much sense. But if you already know what pace you need to go, are trying to win the whole shebang, and are just trying to build up strength and endurance by practicing under more difficult conditions, it might make sense.