You are right. The reason for the three threads is to stop the spammers from swamping the forum. We have tried other anti spam protocols but not worked 100%. Since we implemented this three thread rule the amount of spam you the user is exposed to is basically 0%.
While it can be a pain in the ass initially to post and then have to wait the long term benefits are you don't have to wade throguh Nigerian Princes asking for help, weightloss ads, real estate scams and boatloads of porn.