yes hasSubWikis() is much better, even though its implementation should probably be a one-liner: return getWikiReferences().size() > 1
It's ok, since it's in api.XWiki it's going to disappear in not long I hope. I think this API may need to move in a rewritten xwiki-platform-wiki-manager module that will be a convenience layer above the new model to perform multiwiki operations.
Actually maybe we won't want to draw this dependency in single wiki mode and have the ability to get this information directly from the Model. Maybe modelContext.getServer().getWikis().size() but since this is an Iterable it would mean sending a query to the DB which isn't that nice. An alternative is to have some other module using the query manager to do some DB queries and cache the results to get the list of wikis. However including multiwikis in the query manager is probably too hard and thus another query module for wikis is probably going to be needed. Something like query-manager-wiki.