Development

Show me a browser that doesn't render them the normal way, though.

I believe the original reason was to use them because they actually have semantic meaning, too...
Nontheless, I actually agree, people use them for formatting mostly, not because they care about semantics.  We can change it in 2.0.

I can't give an example, but it doesn't change the fact that it's technical wrong.

Tags <i>...</i> and <b>...</b> are neither obsolete nor discouraged nor impaired or prohibited in both HTML 4.01 Strict XHTML 1.0 Strict.
<i> Indicates italic
<b> Indicates bolding
<em> Indicates highlighted (emphasis added)
<strong> Indicates a set highlight stronger (capacity)

Contrary to what some would have us believe, each of the above tags is valid XHTML 1.0 strict.
Where the worries and confusion begin is that browsers have generally adopted italics for <em> tag and bolding for <strong> tag. However, this conventional choice of browsers not based on any established rule: in the future, they may well decide to display <strong> with enlarged character size and <em> with a left indent, or anything else.
The argument is that the tags <i> and <b> provide no meaning (semantics) to document unlike <strong> and <em> elements that are highlighted. But we must understand that italics and bolding does not necessarily mean that the text should be more important. If this is often the case, we should not think that the tags are equivalent.
That is why I will continue to use, in some cases, <i> and <b> tags and not replace them systematically by <em> and <strong> tags. These four tags have various functions depending on the context.

I would see it in FluxBB 2, but is safe to use HTML tags as bbcode?

This is about the HTML code that replaces the BBCode during parsing.

I know what it is, so I'm not dumb. But is it safe to give html bbcode

No, it is not safe to allow HTML in posts.
What I meant is that it should not always translate BBcode tags [ i]...[ /i] by HTML tags <em>...</em> and so on for tags ....
<i>, <b>, <em> and <strong> are four separate entities.

you could remove [ b] and [ i] and add [ strong] and [ em] but that's not very user friendly ( since everybody expects to use [ b] for strong and [ i] for emphase )

I stands for italics. Anyway, that's not the point, the b-tag in BBCode should be represented with a b-tag in HTML, as the BBCode b-tag means bold., not strong Idem dito for i.

then if the usage for old italic is replaced by blue dashed underline, you want every fluxbb installation to replace all their [ i] tag by a new [ blue-dashed-underline] tag which would be replaced by <em>

and if html5 comes arround ( which add semantic to <b> and <i> tag ), you will use <span class="italic_format"> as replacement instead of <i> ?

Changed my mind. Using strong and em is helpful in terms of accessibility.