It used to be that the rules of the RNC forbid the party from excluding anyone who declared they were Republican. Candidates could be excluded for not meeting 1% threshold in 5 national polls, or not having even formed an exploratory committee, but not because a candidate is wacky. The public decided if a candidate is wacky. The rules still don't allow the RNC to exclude the wacky, but they now have to tools to change the rules and exclude the wacky if they want to.
Some major changes in the rules happened at the last convention that curtailed the power of enemies of the GOPe - what some call "the grass roots." The agitators. The guys who won't follow the rules. On August 23 2012, with Boehner and Preibus presiding the RNC changed the rules to make it impossible for a delegate in the Presidential primaries to vote for someone they were not pledged to. More importantly the GOPe also gave themselves the right to make rule changes outside of the once every 4 year national convention. The old rule makes little sense in this day and age.
The full convention can amend the rules next convention if they want, but I see nothing but good things coming out from the RNC rule changes since 2012. Shortening the primary season gives an advantage to serious candidates with the ability to attract a majority of voters, increases the time the GOPe can raise funds for the general election, and shortens the primary season for the agitator groups like SCF, Freedomworks, C4G, and others.