There are actually good reasons to break the general typographical rules and use uppercase numerals in technical writing.
In usual texts, uppercase numerals are considered ugly or negatively affecting the readability as they form one block without ascenders or decenders and as they stand out from the text and are more difficult to read (for the same reasons that all-caps are more difficult to read). However, the properties of uppercase numbers are often desirable in technical writing:
You may actually want numbers to stand out.
In a formula, you usually want a number consisting of more than one numeral to be perceived as one element at first. For example:
x = 1234 + 5678
Here the first thing you want the reader to see is that x is the sum of two numbers and not the exact values of these numbers (which would be more emphasised with lowercase numerals). Also, from a readability’s point of view, most formulas are a chaotic mess – using lowercase numerals would only add to this.
There are several mathematical notations that do not mix well with lowercase numerals, such as indicating repeating digits in a decimal fraction with a bar above the number.
You usually would not want to use lowercase numerals for super- and subscripts, as it makes it more difficult to recognise whether something is a super- or supscript or not. (Note that this is not the same as for lowercase letters, as they are easier to distinguish from their uppercase counterparts and exceptions, such as the letter o, are usually not used as variables for that very reason.)
Using lowercase numerals for axis tics would make them optically less regular and thus be more emphasised, which is usually not what you want.
In tables, uppercase numerals help outlining the rows, while lowercase numerals obfuscate the structure. (On the other hand, lowercase numerals may make some aspects of the data easier to recognise.)
While you could use uppercase numerals for tables and figure legends specifically, there is a gray zone between
- formulas in which lowercase numerals do not work well
- formulas in which lowercase numerals are no problem
- just numbers.
Therefore using lowercase numbers not at all or only in a few special cases like page numbers or affiliations is arguably the only way to achieve consistency.
A similar thing applies to super- and subscripts: While in normal writing no consistency issues arise, as a super- and subscripted numbers is always super- or subscripted, the semantically same number often appears in normal size as well as super- or subcripted in technical writing. Therefore using uppercase numerals only has a larger impact on consistency than in normal writing.