The theming bonus as it is has had one interesting effect on my gameplay -- still going through my first playthrough, having just finished Uffizi last night I was dabbling with my great works to see what to put where and figured out how the theming bonuses worked. I've already got the Sistine Chapel (currently with two Renaissance Great Works from my civ for its theme bonus), but since Uffizi wants
three Great Works from the same time period from the same civ, I checked to see when my next Great Artist would pop up -- about 13 turns. I then realized that, since I was currently researching Archaelogy, I'd end up in the Industrial era before my next Great Work appeared. So I switched my tech research to other Renaissance techs until I get my Great Work, so I can fill up Uffizi instead of the Sistine Chapel. Perhaps not the most optimal choice, but I'd rather try and get the 3 Renaissance works now, rather than gamble I can get them all in a future era.
I'll admit, I was originally expecting each work to have a certain "theme" assigned to it, like "war" or "nature" or whatever, and you'd get bonuses for collecting them together. But the system as it is does force you to give a bit more thought as to exactly when and where you use your Great Writers, Artists, and Musicians, as you may want to produce their works within a certain time frame.