One factor complicating site initialisation is that Request._add_defaults loads Siteinfo if action=query to determine whether to add rawcontinue.
As mentioned on wikitech-l, that should be fixable by always adding rawcontinue if neither continue or rawcontinue are present, and Pywikibot explicitly not supporting the 1.25 branches where this caused a nasty warning.
To be honest I don't see much of a benefit. Do all versions we support also support the rawcontinue parameter? It also caches the siteinfo so if later another siteinfo (e.g. namespaces) are needed, it would load the version anyway. Also what do you mean with “nasty warning”?
Okay 1.19.24 doesn't complain about unknown parameters but at least 1.21.10: http://www.eierwiki.de/api.php?action=query&titles=Main%20Page&meta=siteinfo&rawcontinue=