There's an interesting post at "Early Modern Notes" on the issues with the label 'early modern'. I'm not sure I have any clear (or even consistent) view on the subject yet. History of philosophy is a peculiar sort of historical discipline; it is the only historical discipline I can think of which is itself, essentially and necessarily, a form of the thing it studies. All history of philosophy is, necessarily, a way of doing philosophy - the question can never be whether one will do philosophy by doing history of philosophy, but only whether one will do it badly or well. This leads to additional peculiarities, and I think a number of them have to do with how we divide up history. The primary trouble with labels in history of philosophy is not, I think, historical at all; historical labels potentially interfere with good philosophy, which in turn can make for bad history of philosophy. The reason is that labels can isolate a period and lead to putting blinders on. All historical change is continuous; even climactic events and catastrophes fit flush with what precedes and what comes after, in the sense that they can't be isolated from them. This is especially true, I think, in history of philosophy. In this sense there is a sort of expendability to the labels.
On the other hand, the 'early modern' label is also in a sense objective and real, a bit like national labels and boundaries are. National labels and boundaries are both arbitrary and imaginary, since they are simply made up by human beings, but they are objective and real in that, if you fail to take account of them in history, it can lead to distortion in your account. The divisions between 'early modern' and other periods are in this sense imaginary and arbitrary, and have all the vagueness and merely-approximateness (to coin a barbarism) that follow from being so; but, in history of philosophy, at least, they are real. For instance, philosophically speaking, in the early modern period the people involved explicitly see themselves as making a break with the past; there is a distinct period to which one can apply the label because the people involved in the history deliberately made the period as distinct as they could from the previous periods. There are a number of other signs - quick shifts in patterns of influence, topics discussed, descriptions of reason and philosophy, etc. The break isn't sharp; it varies from country to country (in Britain 'early modern' can reasonably be said to last well into the 19th century, but in Germany one can reasonably say it ends before the end of the 18th); you can use different criteria for where you put the endpoints (e.g., at the publication of Kant's Critique of Pure Reason in 1781, at the French Revolution, there's even an argument to be made that it should end with Hume's publication of A Treatise of Human Nature in 1740); and it will vary from historical discipline to historical discipline (I've given common suggestions for history of early modern philosophy). But the (relative) suddenness of the change is real.
Further, even in the slowest and most gradual historical change (I suspect this would in some periods be exemplified by, e.g., the long-term stable changes in the socioeconomic conditions of peasants), there can be real differences that need to be distinguished out. The visible spectrum shades continually from very red to very purple. Color terms are notoriously arbitrary in how they divide the spectrum. But red is still very different from purple, and it's useful to have a way of speaking more precisely about the differences, even if the exact divisions between terms are picked out at random. So I suppose my thought is that the 'early modern' label is more than a convenience - but less than a real name. It doesn't pick out anything very distinct; but its value isn't in doing that anyway, but in doing something else, namely, making it possible to talk about the history in the first place.
Or something like that!
