My father Jerry Kopel served 22 years in the Colorado House of Representatives. He represented part of northeast Denver, as a Democrat. Among the posts he held were Judiciary Committee Chairman and Assistant Minority Leader. (His website is here.) He did read every bill before voting on it. Sometimes he was the only legislator who did so; at other times during his tenure, there were a few others who did so, including Republican Tim Foster.
For 18 of the 22 years, he was a member of the minority party. By actually knowing what was in the bills, he was able to offer amendments to improve the bills, take out over-reaching provisions, and so on. More importantly, because he knew what the bills contained, he was not dependent on lobbyists to describe the bills to him. This was particularly important if the lobbying power on one side of a bill was very lopsided.
For example, in 1990 a bill to significantly expand Colorado’s already-bad civil forfeiture laws was introduced. It passed the House Judiciary Committee 12-1. (Although my father served for many years on the Judiciary Committee, by that time he had switched to the Business Affairs Committee.) The weekend before it was due to come up on the floor of the House for a vote, he read the forfeiture bill. Speaking on the floor of the House, he showed the legislators that the bill was far more onerous than its lobbyists had claimed. The bill was defeated by a solid bi-partisan majority.
The Colorado Constitution requires that each bill shall only concern a single subject, which shall be clearly expressed in the title. This provision is sometimes stretched to the limit (and beyond) with broad titles such as “Concerning criminal justice” (the typical title for the District Attorneys’ annual omnibus wish list). Even so, the single subject rule does help make legislation more comprehensible for citizen legislators.
One other data point on reading bills: When the NAFTA legislation was moving through Congress, Ralph Nader challenged the Senators and Representatives to read the bill, because, Nader said, reading it would change their minds. Colorado Republican Senator Hank Brown responded by reading the massive bill. As a result, he said, he changed his mind, and voted against it.