Yes but don’t you think that if somebody deliberately chooses to disable the default CSS, the person actually knows what he is doing and is capable to provide IE8 fallbacks if needed?
In fact, if somebody decided to use “display: inline-block;” instead of floats, or perhaps uses the extended margin syntax (“margin-right”, “margin-left”, etc) it will make his life harder since he also got to know how to “revert” the inline styles applied.
To sum it up, in my opinion there is nothing wrong with including the basic css + inline styles for users who chooses to keep the “default style” but in case somebody chooses to deliberately disable the default styling, it shouldn’t add any styles at all.
If somebody non-technical wants to make small changes, they have the customization box, which is separate from the default styling radio buttons.
P.S – its a simple matter of adding an IF statement to check if the “use default styles” have been set to “no” and then make sure to prevent the inline styles to be included with wp_head() / wp_footer().