This is an issue many Japanese learners come up against, particularly if they live outside Japan. After all, If you are not in the country, it seems difficult to justify the huge amount of time that learning Japanese characters seems to require.
So what are the problems in using Romaji (Roman letters, like the characters you are reading right now) for studying Japanese:
1. Mother-Tongue Conflict
The associations your brain will create between Japanese words written in Romaji and English words (or those of your mother tongue) greatly increases the risk of mispronunciation. Japanese symbols will have none of these associations for you. The very fact that they are completely alien helps you to start your language learning experience from a blank sheet. Your chances of being able to gain the correct pronunciation soar.
2. Show Me The Romaji
Your textbook may be in Romaji, but you will be very hard-pressed to find any real examples in Japan. Of course, you can see a fair amount of reasonably understandable English, but not Romaji. And watch what happens when write some Japanese in Romaji and show it to your native speaker friend: They have a really hard time deciphering it, because Japanese people just don