> it can be a small problem, I think, when "non-random" sequences are removed from possible random number generations. [...] it may take a fair slice out of the available keyspace
This is true, and could be a problem if everyone's PIN were randomly generated. Since most PINs are selected by users and conform to a known, decidedly non-uniform distribution, this actually makes sense. If it's known that e.g. 1234 is over-represented in the pool of PINs, that would be one of the first ones an attacker would try. Therefore, it makes sense to filter that out. But note that it's the over-representation of the PIN and the fact that attackers are aware of this skew that makes it worth avoiding, and not anything inherently insecure about "runs" or "pairs".