The [ ] brackets and the $eval() identifier are not related and share no common code. The [ ] brackets were added very early on in the scripting language's development to solve a specific purpose. They use their own parsing code that is separate from the rest of the scripting language that continued to evolve around them. As with all scripting features that have been in place for a long time, their behaviour cannot be changed, as that could break backward compatibility.
i think it could be a good feature; anyone who doesn't want to modify their scripts can use an older version of mIRC!