Strider,
     I hand a old teammate who did this because of injury, rather than just to try it. He was a 210-220 RH bowler before injuring his wrist. He turned around and bowled LH for at least 5-6 seasons before going back to RH. He started out a little below your level LH, he hadn't practiced it all. He started out very much as a beginner LH. But within 1-2 season he was averaging 195-200 LH. I believe his knowledge from bowling RH accelerated his learning curve LH, but easy THS was also a big part of that as well. I do not know if he went to Nationals as a lefty, or what his scores were if he did.

    I mention this because I agree with your points. An easy THS turned a absolute beginner bowler LH into a 200 average bowler in a very short time. Again, his wasn't a beginner to bowling, so I am sure that helped. But if he were bowling on sport or challenge patterns, I am sure he wouldn't have made such larger strides so quickly. I think your post illustrates the frustration a lot of us have with bowling right now. The THS patterns that are out there inflate averages. The better bowlers, for the most part, realize this and don't get caught up in averaging 230+. But the bowlers who average 210-220 on THS usually don't see it that way, from my experience. There are a few that do, but the majority believe that is an accurate view of their skill level. Then they bowl on a challenge or sport pattern and average 160-170 because they spray the ball over a wide area and now it doesn't hit the pocket. They are decent spare shooters, but they tend to hook the ball at all their spares, so on the harder patterns they are leaving more multi-pin spares and then missing them because they can't throw straight (or straighter) at them. And then they get frustrated and angry and complain about the shot. That is the issue, instead of accepting that they aren't as good as their THS average shows, they get upset and complain and vow to not bowl in that house/tournament/event again. So no one can run a tournament anymore and put out a tougher shot, because then they lose entries and can't sustain it. I don't know the answer to fix it, but if we don't fix it, it means the end of the "sport" of bowling.

I think Strider is over simplifying why guys who score on house shots struggle at nationals.  It is not about having strong or poor fundamentals.  Of course a bowler with poor fundamentals ( I assume by poor you mean lack of accuracy and speed control ) is going to struggle on flatter patterns.  However most guys who score good on house shots and struggle on tournament patterns have good fundamentals.  They simply are throwing too much side roll and have become addicted to swing area that is not available on a tournament pattern.  When you cant get comfortable playing more direct, you are often going to struggle no matter how consistent your release.   

Combined with the lack of sport shot experience.  The pros make the sport shots look easy because that is all they bowl on.

I disagree with Strider on the fundamentals.  If a bowler is accurate with good ball speed and a solid release you cant argue with his fundamentals regardless of whether he is a cranker a stroker, or a two hander for that matter. 

The greatest weakness of the game of bowling, in my opinion, over the past 60 years is that every change in lane surface ( shellac to lacquer to urethane to synthetic ) has promoted a different type of release as optimal, and has forced some pretty solid players out of the game at the highest level.  I first experienced it when the harder finishes of the seventies made great strokers like Stefanich, Neff, Ritger and others obsolete, and relegated their style to being niche players.  They were replaced by cup wristed open shouldered guys who played strike or no count as they heralded in an era of designer splits using styles we considered as fundamentally weak.  However, out of this era the power game evolved to the point we see it today where power players are good spare shooters and have the versatility to play straight or hook the lane. 

In the sixties there were a lot of guys who were good spare shooters, and accurate, but simply didn't throw enough ball to compete at the highest levels.  In that environment the strokers who threw the most ball were still accurate.  Did that mean the straighter players lacked good fundamentals? 

I very nearly quit the game in the late 80's because I was unable ( or unwilling ) to adapt my strokers  style to the short oil game that predominated.  The return of long oil followed shortly thereafter by the introduction of the resin enhanced urethane ball rejuvenated me and made me relevant at a high scratch level again.  My fundamentals didn't improve any more in 1990 than they deteriorated in 1984.

In a classic case of getting to soon old and too late smart, I finally modified my release in about 2006 to incorporate the so called less is more release and stop hitting up at the line.  Again once I became more proficient at this release I was able to borrow more time for myself at the competitive level, but also, again, my fundamentals neither improved nor regressed as my new released allowed me to throw a more effective carrying ball.   

I think spmcgivern said it better than I did.  I was getting more at the guy with the chicken wing wild arm swing who can still score pretty good on a THS.  Weak fundamentals, but can repeat shots pretty decent.