First of all, let me say that I'm in favour of the idea and would make extensive use of it, particularly if you were allowed more than one variation on each listing.
But...
There's much more to it than just adding a couple of dropdowns for variables. A size variance may also affect the price of the item so the users would then have to provide higher or lower prices in relation to the default item (and then remember to change them all if the default size is discontinued and deleted). For larger items a size variance may also affect weight, which could in turn potentially affect postage. And then you need to provide variable stock levels for each item. And so on...
Presumably the entire site is built around not having this option, so although it shouldn't be impossibly hard, it'll be a lot harder than most folk would think.