Sometimes people want the ability to tell which k of n signed (for accountability purposes, if k < n) and there a multisig has additional functionality that schnorr (or other) mulitparty sigs dont.

I did also mention on twitter another idea to use schnorr to make a compact multisg.  The idea is  to have enough different keys per signer to make it unambiguous which k signed, and commit the public key sums for the needed permutations in a merkle tree.  Then to sign reveal the path to the public key sum used and a multiparty sig with that (via the usual schnorr method).

It seems that Micali et al thought of this idea before see http://www.cs.bu.edu/~reyzin/multisig.html though the numbers sound better for them as they're assuming prime fields not EC where a hash of a public key is a smaller ratio than in EC.

I expect it wouldnt be too useful to use the bootstap method I mentioned earlier because the subset doing the bootstrapping know everyone elses keys and so could impersonate them for accountability purposes.  So you do need an enrollment process which does not involve third parties knowing parties private keys!

Adam