Prior to version 5.91, $addtok(foo,$null,32) would return "foo $null". From 5.91 onwards, nothing is returned at all. Presumably this behaviour is a result of the following note in versions.txt:

15/06/2001 - mIRC v5.91
(…)
37.Improved the way $null is handled in scripts.


Anyway, I agree that this isn't logical behaviour.
$deltok is also effected; $deltok(foo bar,$null,32) should sensibly return “foo bar” i.e. Take nothing away.