From the day EVO owners got treated to their new super-phone, they have had open access. But while it was rooted, it wasn't full access. (the nand protection hadn't been unlocked which denied full read/write access while booted within Android)
That protection prevented features such as microSD card-enabled swap and apps to SD from being used. But as of now those days are over. The folks you can thank for the original root method, Toastcfh and friends, have finally gotten complete root access. There may not be any ROMs available that can take advantage of the increased capabilities, but just give the Android/EVO dev community time and they will appear.
Until they are released however, the benefit will just be to remove the need to use ADB to boot into a custom recovery. Next step is to get a hold of the devices original kernel source, but until HTC releases it, the fully unlocked experience won't be available. Check out XDA for the latest.