@Vincentt: concrete example: Emacs. By default, Emacs indents code automatically based on syntax; it will line up function parameters with the opening parenthesis if they need to extend over two lines, and it will try to sensibly indent things like statements that continue over two lines. As a result, things are not always indented a tidy 2, or 4, or 8 characters; they’re often indented 17 characters, or 9, or 14, because function names are not always a tidy 2, or 4, or 8 characters long.
So what happens when Programmer A, writing Perl in Emacs, saves his file? Emacs looks at the tab width it’s programmed to use (by default, 8 spaces), translates the 17-character indent to 2 tabs plus 1 space, and saves that. The 9-character indent becomes 1 tab plus 1 space. The 14-character indent becomes 1 tab plus 6 spaces. (You can turn off this space-to-tab translation, and any programmer working in a homogenous environment will almost certainly have done so.)
So then Programmer B fires this up in his editor of choice—say, BBEdit, which has 4-character tabs by default. Suddenly all the indentation is off. Function parameters no longer line up with the initial parenthesis; statement continuations no longer are tidily indented past the earlier statement. The code looks like crap.
So, practically speaking, if you’re working in a group with me, you have a few options. You can force me to use something other than Emacs (and watch my productivity plummet as it takes me six months to finally find and eradicate all the annoyances in the mandated editor, and it still won’t do everything Emacs does); you can write a clear style guide that specifies exactly how things are to be indented, and then implement it in Emacs cperl-mode (you are the advocate of tabs over spaces; you get to do the heavy lifting); or you can save your work with spaces, and I’ll save my work with spaces, and the indentation will work across editors. This third option is significantly less work and requires very little advocacy.
No less a luminary than Jamie Zawinski has a similar opinion: http://www.jwz.org/doc/tabs-vs-spaces.html
If you aren’t compelled, hey, use the tabsl. What you do in your code is your business until I have to work on it, and if you’re writing in PHP, the chance of that happening is virtually nil.