Think of this like a question of how much salt a human should eat a day. If you do it "right" and limit your salt intake to USDA guidelines you reduce risk of stroke, heart disease, hypertension, and a fair number of other diseases.
The people who are saying to do a 50% water change every week are doing the equivalant of making sure your betta doesn't get too much salt (except that salt in low quantities in needed, and a betta would be fine with no ammonia, nitrates or nitrites). Yes, this is probably the absolute best you could do. However, being less than perfect in this is not going to kill your fish out of hand. Having twice the daily recommendation on salt isn't going to kill you out of hand either, but it might make you less healthy.
I would feel comfrotable missing an occasional water change if I were doing the 50% weekly, because I know that I'm otherwise keeping the betta in optimal health. If I were doing them every other week, I wouldn't expect the betta to suffer any immediate effects, and think that the betta would *probably* live a perfectly healthy life.
The advice you get here is how to keep your betta in tip-top best possible shape. Falling just short of tip-top is still really good.