[Lucas89]

Edit: beaten to it

you don't really need to memorize all of the seperate combinations of kana since if you just learn the pronounciation of the individual kana you will know how to read it anyway, since the sound doesn't really change.

Lets take for example イェ from that chart on the link you provided (I'm assuming these are the katakana combinations your talking about).
you will learn イ as an ee sound like in the word feet,
and you will learn エ as an e sound like in the word enter

now say both those sounds together and you get イェ, which completely eliminates the need to memorize all of those seperate little combinations.

Not very good examples i know but you get the picture.