The second problem is that IE is going to hold you back in what you can do, preventing you from doing anything beyond the most basic CSS. Instead of allowing you to use all the spectrum of what's possible and then give you the option to find in the end something that looks OK enough for these legacy versions.
Moreover IE constantly sets you on the wrong foot, slowing you down significantly.
Mixing in workarounds for legacy IE versions in the main CSS is also counterproductive in the long term as it encourages browser crafters to emulate the bugs in old versions IE.
The worst of it all: dealing with all the IE bugs just drives you nuts.