Why are beginning to intermediate bridge players told to delay learning how to bid certain types of unusual hands?
To best understand the answer to this question, it helps to understand the purpose of bidding and bidding systems. Bidding is an attempt by two partners to predict the number of tricks their combined hands can win in the play. The purpose of bidding is for each partnership to ascertain which contract, whether made or defeated and whether bid by them or by their opponents, would give the partnership their best scoring result.

A bidding system is the set of agreements and understandings (treatments and conventions) assigned to bids and sequences of bids used by a partnership. As a beginning player, you might think it a simple matter to develop a bidding system which, if followed, would always land each partnership in the contract which yields the highest score. In reality, no perfect system has been developed. The problem is that the number of possible hands is too large to be mapped onto a tidy bidding system that works well for all of them. There is simply not enough bidding space to develop such a system.

Since designing a perfect bidding system is (probably) impossible, compromises must be made. The way this is usually done is to divide hands into types and develop a sense for which types of hands are most common, less common, uncommon, and extremely rare. When designing a bidding system, you make darn sure that the most common types of hands are easily dealt with, landing you in the highest scoring contract 100% of the time or close to it. Then you keep modifying the bidding system to deal with ever less common types of hands.

A beginning to intermediate student of bridge will do well to follow this same path of successive levels of approximation.

For example, if you're learning the Standard American system with a pair of books by William S. Root (Commonsense Bidding, Modern Bridge Conventions), you first master the bidding basics in the first 10 chapters of the book Commonsense Bidding. Perhaps that will enable your partnership to handle 65% of all hands once you've thoroughly mastered these chapters. Then you read the rest of the book to get your partnership able to handle 86% of hands. Then you cover the one-star conventions in the next book, Modern Bridge Conventions, to get you up to 93%. Then the 2-stars get you to 96%. Then the 3-stars get you up to 97%. Beyond these 2 books you'll get into increasingly esoteric conventions that come up so rarely in play that they're very hard to remember - but may help you tackle most of the remaining 3% of hands not covered by the treatments and conventions described in these 2 books.

The actual percentage of hands covered is almost certainly different than what I just described, but the concept is valid and can be applied to learning:

The most efficient way to learn bidding in bridge is to concentrate on the most common hands first and master the treatments and conventions for that thoroughly before starting to move on to deal with successively less common hands.

A common mistake of beginners and one which I made many times is to look at a certain hand and realize that the bidding I've learned so far probably won't work well for this type of hand. So instead of doing my best to follow the treatments and conventions I've learned so far, I simply ignore the system and try to do what feels right for the hand. This not only results in many misbid hands (your partner has no idea what you're doing!) but also slows down learning how to bid if you do this frequently.

Another common issue is getting obsessed about learning how to bid a certain type of hand. Yes it's very cool to learn how to bid slams that have a missing ace but a void in that suit (Splinter bids). But this type of hand comes up maybe once per thousand deals. If you try to learn that before you've mastered the basics, you're concentrating your efforts on a 1 in 1000 occurrence before you're dealing well with types of hands that come up every time you play.

So that's why you want to delay learning how to bid certain types of hands even if it lands you in poor contracts from time to time. By learning how to bid more common types of hands first, you make more efficient use of your learning time and get more used to applying basic treatments and conventions. Once you master the basics, you can better refine your bidding system with added treatments and conventions but if you don't have a good foundation to build on top of, this is more likely to hurt than help.

I know from experience as I had to unlearn years of bad habits developed during casual play - habits that would not have formed had I understood this concept of successive levels of approximation. Once I understood this, I then fully unlearned my bad habits, and my bidding improved dramatically.