Both of those reasons are pretty much stale nowadays. Contemporary implementation where conditions are standard objects are CCL, ECL, Clasp, ABCL, CLISP, SICL, LispWorks, and Allegro, leaving out pretty much only SBCL (which does this for bootstrapping reasons, such as having errors much earlier than CLOS) and probably Genera (which is hard to call contemporary). And AFAIK there are no alive subsets of CL whatsoever, let alone those which have a condition system but do not have CLOS.