Dr. Virago posted a couple of times about seeing the phrase "medieval Catholics" in scholarly books ... something I mentioned is a real pet peeve of mine. I've really got two reasons for this, one scholarly and one spiritual.
The single most important medieval technology for inventing the modern world was the printing press, and one of the things the printing press allowed for was Protestantism. Medieval Christianity is a manuscript culture religion, and as such, while Protestantism could be considered in theory, in practice lack of access to Scriptures meant that it was impossible. If you wanted access to Christian scriptures in the West, you had to do it through someone with access to a Bible and the ability to read it.
In spite of (Because of?) that dynamic, you don't have strictly unified proto-Catholic Church through the Middle Ages. The various religious orders wax and wane, as do movements like the Benedictine Reform, the conflict between competing Church heirarchies (such as Bede describes between the Irish and Roman Church), and monasteries move from being lay organizations to clerical organizations.
If there were any proto-Protestants, they were the Lollards, but though the term technically refers to a specific religious movement, it tends to be used by medieval people as a catch-all term for heretics, especially anti-clerical heretics. Though there are some scholars who see Protestantism as growing out of Lollardy, I think that notion is in the minority.*
The idea of being "Catholic" with a big C is one born of the anxiety of the modern era. We could say that the Roman Catholic Church solidifies in the modern era in part because of competition from Protestants. Lollards were never a real threat, but Protestants were. In some ways, the Catholic Church strongly centralizes power to oppose Protestants, but in other ways, it is to respond to some of the more legitimate complaints of reformers by instituting a sort of "quality control," much as the aptly-named Gregory the Great did in his day.
My objection, then, is that the term "Catholic" is really anachronistic in this setting. To offer a parallel, it would be like calling medieval clocks "medieval non-digital clocks." It's absurd to draw the distinction because there was no such thing as a digital clock in the Middle Ages. To say "medieval non-digital clocks" implies the existence of medieval digital clocks -- and short of some really fantastic find of an LCD calculator watch buried for centuries in a bog somewhere, I think we can say with some certainty that there was no such thing.
In the same way, the phrase "medieval Catholics" implies that there was such a thing as medieval Protestants -- but there wasn't.
As I complain about here, a lot of Protestants want to discount the medieval Church as being the flyover country of religious history, with the First Century on one coast, and the Reformation on the other coast. In this view, nothing of much importance happened, and what little was of importance was bad, such as Crusades and the Inquisition** and whatnot. A lot of Catholics also like that distinction, because it allows them to say the equivalent of "Ha! We were the Church back when you Lutherans were still living in caves!" -- a bit of an exaggeration, but in that spirit.
This is bad for the Church generally. With each side saying "We are the civilized ones, and you are the barbarians," each side can discount large patches of Church history. That tension and rivalry doesn't weaken the Church, though ... it strengthens it.*** Reading the Acts of the Apostles, it is pretty clear that the early Church was full of conflict. To avoid this conflict by disowning the other strikes me as a pretty un-Berean attitude. Paul and Barnabus fight over whether to take Timothy along with them, and they fight so sharply that they go their separate ways, but neither one says anything like "You aren't even of the faith any more and I'm going to ignore you" -- and a good thing too, since Paul changes his mind about Timothy later.
At the end of the day, the phrase "medieval Catholics" can serve as a way for Protestants to disown the medieval Church, or it can serve as a way for Catholics to exclude Protestants from the common medieval Christian heritage. It's a shame, really, since the medieval Church has much to teach us all.
*That being said, I think it is acceptable to use the phrase "medieval Catholics" if you mean to distinguish them from medieval Lollards.
**Just a reminder that the Spanish Inquisition was actually a modern event, not medieval.
***And here's where I get all the angry e-mails.