I don't wish to dig up a holy war, but why would you want to use spaces instead of tabs? That's what they're for, isn't it? One tab = one indentation level. No need for messing around with spaces, which serve other purposes.
Once you have mixed tabs and spaces, everything goes to hell because now that code's formatting is tied to a specific person's idea of how many spaces should a tab display as, or whether tabs refer to tab stops or just a fixed number of spaces.
I never had strong feelings about it until encountering some really pathological code bases, which had gotten to the point where there was literally no valid tab->spaces setting that would make everything look correct.
Ultimately, though, these things really just don't matter. I use 4 spaces per indent level, but it's not like I'm incapable of reading and writing code that uses tabs, or 2 spaces, or 8 spaces. Really the only thing that matters is that there is a standard and that it is consistently applied within a project.
So, if you indent something with 4 spaces, then other people viewing your code will also always see 4 spaces, therefore ensuring that whatever you found to be readable indentation, does actually come out like that on the other end.
Tabs are accessibility. People use spaces for the same misguided reasons they use px font sizes in css.
(Rant over. Yeah, this annoys me.)
I'm sure there are other reasons too.