For example, one person has tabs set to the size of 8 chars and writes a lengthy script. A second person comes along with tabs set to 5 chars and makes changes. The indentation could well be wrong because they see the same file differently. Sometimes you get lucky and you get errors immediately, but other bugs can lurk... like the second statement under an if block for example. Should it be included in the if or not? Bug.
Therefore, tabs aren't worth the trouble they can potentially cause. Never mix them with spaces either.
Also, using spaces is not difficult in a modern editor. You don't have to actually type four spaces each level. You set tabs as 4 spaces in the prefs and then every time you hit the tab key you get 4 spaces instead, and when you hit the backspace it removes 4 spaces (geany does this).
Whatever you use, I suggest turning on the display of whitespace in your editor. It will then show tabs as "--->" and spaces as a dot. Change the color in the highlighting preferences to be very subtle so it does not overwhelm the code. It will help you keep every file you work on consistent and such errors a thing of the past.
Plus, each developer has the freedom to choose how many spaces a TAB is - I like 8 when I'm using my big monitor, but I prefer 4 on my laptop. With TABs, I can see it both ways depending on the machine.
On the off chance that I'm ever actually editing in a smaller window, I can even set it to 2 characters instead, but when I open that file again on another machine, I can see it as 4 or 8 characters.
It might not be the case for everyone, but my laptop screen is 1920x1080 and has more room for text than I can use. In a three way merge your strategy could help me perhaps.
(1) Some editors will autoconvert them to spaces. Every time you switch editors, switch IDE's, edit your code on a different computer, or let someone else work on it, you have to remember to make sure your tabs are still there.
(2) With tabs, your code will look different for different people. I often line up parts of adjacent lines that are symmetric, and different tab widths will interfere with this. Ditto ASCII art in comments.
(3) It's hard to use tabs consistently because tabs and spaces are visually identical in most editors. (This is less of a problem in Python than other languages, because inconsistent tabs/spaces in your indentation are often caught quickly because they cause syntax errors.)
(4) My mathematical mind admires systems that are designed around simple and elegant ideas. A character display embodies the following two such ideas:
(a) Each location on the screen contains exactly one character.
(b) There is a one-to-one mapping between the pixel contents of an 8x8 box (or however big your font is, assuming all sane people use monospace fonts for coding) and an ASCII character. (Don't get me started on Unicode. I don't understand it, don't use it, and it haven't had it do anything for me but cause bizarre problems [1].)
Tabs break both of these properties. This causes cognitive dissonance and offends my sense of aesthetics.
For example, see set expandtab in vim: http://vim.wikia.com/wiki/Converting_tabs_to_spaces
In most IDEs, a TAB can be displayed as any number of equivalent spaces (8 being the common indent size), so if I switch back and forth between my big monitor (8) and my laptop screen (4), I can view the code with an indent relative to the width of the screen.
If that code uses spaces, I'm usually stuck seeing an indent of 4 on the big screen monitor - something I find irritating because it makes it harder to visually scan and parse code quickly.
So, even in Python, I always use TABs whenever I have the choice. It's just more flexible.