That makes no logical sense and it also directly contradicts the mIRC help file.
[Below from mIRC's help file]

Using brackets speeds up processing. If an alias uses too few brackets then the statement might be ambiguous and the alias will take longer to parse, might be parsed incorrectly, or might not be parsed at all.
The Start bracket { does tell msl that it's the start of the commands but it also says that it is the end of the if/while or whatever. Without the { msl will have to do several evaluations to determine the end of the if/while statement.
And as far as the end bracket } goes, unless you have a blank space after the end bracket, msl has to evaluate the last character anyway. How else could it find the end of the line?

"Excessive brackets" could slow a script down, but 1 set of { } in a if statement will Not.
Also, if you read the whole tutorial you will see that Yochai shows you to take long ugly single lines of code and break them down into several lines of properly bracketed code.
[A quote from Yochai's tutorial]

Alias com-chan { if ($1) { var %a = $comchan($1,0) | if (%a > 0) { while ($comchan($1,%a)) { .var %cc = $addtok(%cc, $comchan($1,%a), 44) | dec %a } echo $nick Common Channels: %cc } } | else { return } }

This looks like a mess, and in a few weeks it will be pretty hard to follow.
Here's a better way of scripting it:

Alias com-chan { 
if ($1) { 
var %a = $comchan($1,0) 
If (%a > 0) { 
while ($comchan($1,%a)) { 
var %cc = $addtok(%cc, $comchan($1,%a), 44) 
dec %a 
} 
echo $nick Common Channels: %cc 
} 
} 
else { return } 
}