A nice-to-have feature is the FCK spell checking capability, native on fck 2.6.4 with support of online service SpellChecker

http://dev.fckeditor.net/ticket/2685 (It is free, but the free version displays a banner space while the spell check dialog is open).
Maybe with EE edition can be found commercial agreements to remove the banner, I don't know...

There are other possibilities to integrate spell checking with FCK: see http://docs.fckeditor.net/FCKeditor_2.x/Developers_Guide/Configuration/Spell_Checker.

I suggest also to consider the new FCK 3.0, near to stable release (Beta 2 out at time of writing). It is not on 2.6.x series, but as far as I know there won't be huge limitation on porting Magnolia FCK integration over 3.0 release of FCK.

Sure native spell check can be a good feature for marketing!