It's perfectly fine to use only half-width arabic numbers.
However, there are other rules in operation, coming from various time in the history of writing and printing:
- A. Don't use arabic numbers at all - maybe seen in formal documents:
- B. Half-width for two-digit numbers, otherwise full-width - mostly in printed materials:
- C. Full-width for a digit, half-width for more than two digits - sometimes used in electronic texts and printed materials:
If you're writing some serious documents, check if there is any style guide provided that specifies which to use when. Or you can adopt one of the publicly available style guides. For example,
All of the three recommends to use half-width numbers in general. So there you have a justification to use half-width numbers only.
[Note: my knowledge is biased towards software documentations]
Traditional Japanese typesetting faced the problem of how to arrange multilingual text that include Japanese and Roman characters in an aesthetically pleasing way, when Roman characters were introduced to Japan.
Take vertical typesetting. When you're writing vertically, there is no way for a word with more than two half-width letters to fit in a single cell without losing readability. They'll have to occupy individual cells vertically next to each other. In fact, Rule B is actually optimized for vertical writing. ex:
When digital publishing entered the scene, these conventions have been carried over to electronic text composition. Consequently, some people use different standards for full-width/half-width numerals as we've seen above.
Side note: Another practice in writing electronic text tries to replicate the aesthetics of horizontal typesetting. That is, inserting a half-width space whenever there's a pair of half-width letter and full-width letter:
空白を 1 つ -
空白を1つ. This is a workaround around the fact that modern consumer softwares don't layout Japanese texts properly. Probably except TeX. Whether this practice is acceptable or not is a favorite topic of Japanese blogosphere that pops up every so often.
- 数字の扱い方 (how to handle numerals) from a company specializing in desktop publishing. Covers historical backgrounds.
- More ways of mixing different character types are covered in W3C's Requirements for Japanese Text Layout, which is based on a standard established by Japanese Industrial Standards Committee.