-u doesn't make sense in a local variable (because of the way mIRC triggers timer events and the single threadedness of script execution, -u can never actually trigger until the alias ends anyway).
Actually, -gu would be rather very handy for cross-alias local variables. Many scripts currently use /set -u %var foo (for things like multiple returns and such) ; simply expending that feature for /var would be a nice addition.
As far as "plain text" goes, just use $(text,0). If you want the features of /set, just use /set.
Not only does that not work (var -s %x $(4 + 1,0) => * Set %x to 5), but for most cases where you actually do need to evaluate it as plain text, you can't really use $(,0) since you want to evaluate things. Of course, like always, obscure workarounds exists. But that's what they are, workarounds.
Being able to use all these features with multiple variables are the entire point of that feature suggestion.
