This suggestion sounds not for the first time... the main problem of it we see is that miniBB is multilingual software by nature, not english-only. We can not include such feature by default, because in that we case, we also will need someway to convert non-english topic titles to latin-like titles when creating SEO URLs. And it will be a VERY difficult task.
In general, this rule is mostly a bogus. miniBB was indexed well even at the times when it has no .html-URLs option; for now, miniBB is also indexed very well without this rule.