I don't think it's okay to take the choice away from the user and for you to decide what the "only good title" is for a subsite - not all multisite setups are the same. It also can potentially make the breadcrumb unusably long when an accepted abbreviation for the subsite's name can be used instead.
In my case, we are using subsites for departments/offices within a university, with the main school site as the main site. If we have Jazz Studies and Contemporary Media as the subsite title, the breadcrumb becomes effectively useless because it has already nearly exceeded the horizontal space allotted. I think just the word "Jazz" would suffice in such a case, especially given the context (header graphic, other visual elements).
If you allow the main site to be labeled differently when that option is filled, then the same should apply for the subsite. I personally can survive with working around it using the Home Prefix instead, but I would love to streamline our site setup process (done only by web admins) and don't see why it would cause any harm to use options that are already set in your plugin.