Bidding $420 because you believe that is what will get you the win is one thing. Bidding $420 because you think it's funny (like $69) is another. One is a person trying to get up on stage, the other is a clown looking for their 15 minutes.
My grandmother, who has been watching Price is Right for 50+ years has always said she thinks they should have a rule of bidding a certain amount within others and I've always said that makes things unfair. If contestant A bid $999 and I know (or genuinely believe) the price is $1000, it's not fair that I would now have to bid something like $1025 and risk going over. It's tough because I also get annoyed when someone one-ups the last person to get called down, a crowd favorite who had been stuck in contestant's row all show, or even someone who is a clear fan vs someone clueless who probably hadn't seen a full episode of Price is Right since it started. In the end, it's still a game and has always been this way .
I can see your point, but what if someone bid/added "420" or "69" to their bid because it actually would get them the win? Some people might think one is being funny, but if it were me, I wouldn't be doing so deliberately for humor; it would just be because I felt that doing so would get me the win if I am the last person to bid and felt that I needed to think critically.
I would concur, however, that bidding those numbers repeatedly to no avail would be rather obnoxious.
Also, you are right that it would be unfair to ban bidding $1 higher...the insane amount of complaints I see from people begging for that rule to be changed only tells me that some people have never seen TPIR in their lives to know the rules of the game that have been the standard since day 1 in '72.