With PubMed Central's cross-journal search engine and hyperlinked references, following a trail of information across medical reviews will be no problem for the researcher. Twigging is notorious because it sometimes produces journals so specialized that their entire subscription base is the two hundred libraries of the institutions where the two hundred researchers in the field do their work. Untrammeled twigging can bring down a publishing empire. On the other hand, twigging allows small specialties to communicate. With PubMed Central, the economic risks are limited.
Space is available on line.
A parent journal might choose to include abstracts of subset articles in its paper edition, and have the full texts on line. Or the subspecialty researchers could start their new journal on line and watch usage. If it is appreciable, then there may be arguments for publishing in paper. If it remains small, then a society or publisher does not incur the costs of floating a new journal in a small market. There are probably dozens of other uses of PubMed Central that are not obvious today.