Threaded cogs and freewheels have been used forever. In the 1980's road bikes changed over to freehub cassette systems, but track and BMX retained threaded systems. Threaded cogs with lockrings work fine when installed properly and good quality parts are used. The vast majority of track bikes that are raced on the velodrome use this traditional system, and riders are constantly removing and installing different sized cogs without any problems. There are a few splined systems, Miche and Easton, but I've only seen a few being used (I do have a Miche setup on one of my wheels that I now use on the street). There is also the ISO bolt-on cog system. Bottom line, if it ain't broke don't fix it.