A few thoughts, first you should learn hiragana waaay before kanji. When learning the writing system you should start with hiragana, then katakana, then start kanji. Kanji is something you'll continue for the rest of your life (just like a native Japanese speaker).  

Of course you CAN learn without using hiragana but most materials for learning Japanese are in hiragana so it might be difficult to find good materials. Also, everyone seems to find this hard to believe but Japanese is much much easier to read in hiragana than in the alphabet. 

Also, writing in the alphabet has a tendency to cause pronunciation problems. There are many different ways to write Japanese using the alphabet. Spelling is very different depending on what method you use. Some of them are not intuitive for learning how to actually say the words. For example, you wrote "konishiwa" and "arigato", but if you read those exactly as they are written it would be wrong. You lose a lot of information about the sounds of the language (mostly long vowels, double Ns, and sometimes it's written using completely different consenents). 

Last thought, hiragana is not nearly as intimidating as it seems. You can learn it in just about a month of you put your mind to it. Maybe not well enought to feel confident in reaing quickly, but that's what practice is for. 

In my opinion, it's best to start learning some words and grammer in the alphabet, while you also start to learn hiragana. Once you feel somewhat comfortable with hiragana, phase out your use of the alphabet. 

I think Niklas added some good information. You cannot write everything in Kanji. Fore example, Japanese children's books are written all in hiragana, they learn Kanji all throughout elementary, middle, and high school.

Another possible problem with learning only in the alphabet I see, is that most romanizations are geared toward English speakers (I think). We use mostly the same letters in French, German, and English but if you put an unknown word in front of each of us we'd guess the pronunciation differently based on our native language. You'll have to become very familiar with the phonetics anyway, attaching them to hiragana is not that much further of a step. Also, think of Kanji more of symbols of meaning, rather than sound. Yes, each Kanji does have set sounds, but often quite a few and you can just learn those as you go. Hiragana and katakana are only sounds though (and the exact same ones at that) so they're much easier to learn.

It's very common in Japanese to borrow foreign words. There's quite a few English ones, some German (like arubaito from the German Arbeit) and some French or Italian (can't think of any examples though). However, they are pronounced very differently from the originals. You start to understand why when you learn their writing system. 

Anyway, I'm sorry for writing so much. Haha this is just a question I see quite often and I actually feel that it's quite important to learn the writing system. I promise it's not that scary (I personally enjoy it very much). :)

I have to agree with Jordan - learning hiragana first is the right way.

I think you've got a wrong understanding of what the three writing systems are about. Sure, kanji might be used very often but you cannot write Japanese in kanji only - but rather you can write it in hiragana only. Kanji are symbols that compress the sounds of Japanese words for the sake of making it easier to read and skim through the text. The most basic reason for using them: to have an option to distinguish between the homophones (so, words that sound the same but have a different meaning) because otherwise you can guess what someone actually wants to tell you.

That's why you should at least be able to read hiragana - not fluently, that comes with time, but for your own sake, to make things easier. Like Jordan said, most learning material is written in hiragana (or have furigana over the kanji, so you still know the reading).

I think it's also good for you to get an idea of writing in hiragana only, kanji and simply latin letters (called romajî btw)

私は後で銀行に行きます。

わたしはあとでぎんこうにいきます。

Watashi wa ato de ginkô ni ikimasu.

-> I'll go later to the bank.

Even without being able to read any kind of Japanese you'll notice that the first version looks, maybe a bit more complicated, but most of all more clearly to look at.

A last thing I'd like to say: Your way of pronouncing "arigatô" is a bit strange. You don't really put a stress on the second a-vowel. The o at the end is long because it's normally written as ありがとう -> arigatou <- ou=ô long o.
An example, where a wrong transcription changes the pronounciation.

I agree with Jordan and Niklas. They have provided a lot of very good information indeed.

By the way, as a native speaker of Japanese, I never think it is "blasphematory" to write Japanese in Latin alphabet, which we call "romaji"! I think it is a good way as long as you are at elementary or pre-intermediate level. However, I think it would be better to learn kanji as well when you try to acquire a lot of vocabulary. It is very hard to study a kind of vocabulary called 'jukugo(熟語)', which is a set of kanji, only by learning its sounds, because each jukugo often sounds very similar. It is much easier to learn it by studying the meaning of kanji at first.

I suppose if you're only interested in oral language there's not much I can say...I don't really understand that. It's like saying you're not interested in half of the language. You wouldn't ever be able to read articles, or chat with people online, or use the majority of Japanese learning tools, or understand signs and notes in movies that might not be read aloud. 

Anyway. Yes, each hiragana has only one sound (there are about three exceptions to this rule). They're very easy to learn. 

In the end, you have to choose your own learning method. If you aren't actually interested in the writing system then you'll see it as a chore and it might discourage you. You already know my opinion, you'll have to decide what's best for yourself.