When you give the player a rare but powerful item which can only be used once but is never really required to proceed, most players will not use it at all, because they are waiting for the perfect moment. But even when this moment comes, they will still be reluctant to use it, because there might be an even better moment later. So they keep hoarding it for a moment which will never come. 

In the end, they will carry the item around until it is outclassed by other, more readily available resources, or even until the very end of the game. That means that such one-shot items don't provide any gameplay-value at all. They are simply too awesome to use.

What can you do to encourage the player to make use of their one-shot items and not hoard them?


Question: How do native speakers 'guess' the pronunciation of the letters in a word they see for the first time?
Answer: Since pronunciation changes over time, any system of spelling that was mapped to pronunciation at a point-in-time would eventually get out-of-sync with the way people are speaking.  

The system of spelling also has to make a choice (explicitly or implicitly): 

a) nominate a "standard" pronunciation, so that from the get-go the system would not map to regional variations in pronunciation; or 

b) reflect regional pronunciation, so there would be many ways to spell the same word. 

The latter was the case before English spelling was standardized. There are dozens of ways to spell the word for 'sister' in Old English, for example.

The Googles of the world would have a difficult time searching if a word might be spelled in a given text in any number of ways. All searches would have to be phonetic, and the search engine would have to understand the rules of regional pronunciation, and to the degree it did not, there would be misses and false positives galore.

It's possible to "get close" when guessing a pronunciation, but that's about it.
If you don't recognize the word as a loan-word from another language, say, and so may be following some special rule, you won't stand much chance of guessing right. For example: depot. But for the most part, spelling of English  words does follow some basic phonetic rules that can be learned.