Rosetta Stone's usage of kana for words instead of kanji


As I use Rosetta Stone to learn Japanese, I only use the Kanji mode (except
when I forget the reading for a kanji and then flip it briefly to furigana
mode). However, I've found that for some reason there are many words that
Rosetta Stone seems to arbitrarily choose to use hiragana instead of kanji.

Examples:

[code]

    ぼく instead of 僕
    こわれる instead of 壊れる
    かさ instead of 傘
    はしご instead of 梯子
    
[/code]

and many more.

Is this due to the kanji being to complex accoring to Rosetta Stone or is this
a situation where hiragana is typically used instead of kanji? From what I've
been able to find in online dictionaries, these aren't flagged as "typically
written as hiragana". Since we're reading these words and not writing these
words, I would have expected them to favor the kanji forms.

What further seems to confuse me is when reading manga, I've seen 僕, ぼく, and
ボク used.




**僕 and ぼく**  
僕 is more common, but ぼく is also used in casual situation.

**こわれる and 壊れる**  
壊れる is much more common. こわれる is acceptable to use.

**かさ and 傘**  
Both are common to use.

**はしご and 梯子**  
はしご is much more common. 梯子 could be used, but it is not recommended.

In general you can refer
[常用漢字一覧](http://ja.wikipedia.org/wiki/%E5%B8%B8%E7%94%A8%E6%BC%A2%E5%AD%97%E4%B8%80%E8%A6%A7)
to find out individual kanji is common or not. If [常用漢字]{じょうようかんじ} is a list
of basic kanji that are recommended to use in everyday life. In other word, if
a kanji is not listed in the [常用漢字一覧]{じょうようかんじいちらん}, the kanji is usually too
complicated to use in everyday life. In that case, you should use Hiragana
instead.

