Lots of people ask this question, replacing Latin with their favorite language. http://www.ethnologue.com/language_index.asp might give a hint as to the size of the general problem. There are about 6800 known languages, in many more language/country combinations.
Latin in unique in a couple of respects, the most obvious seems to be that it'd be a neutral culture, but we always provide specific cultures.
Collecting data for cultures is a challenging task. Since we can't be experts in all languages, we often rely on other sources for our cultural and language data. Often times personal, regional, geopolitical or other differences make it difficult or even impossible to find the "best" choices for a particular culture. I'm glad that you chose Latin because in some senses its simpler and doesn't have some of the geopolitical problems of other cultures.
So what we did do in Whidbey is provide the Custom Culture mechanism so that users can create cultures for their particular needs. Languages and markets haven't focused on can create cultures for their use, historians and librarians could create Latin or other cultures, businesses could create preferred cultures, and Trekkies could even make Klingon if they wanted.
For many of the 6800 languages, good cases for supporting those cultures could be made. I can think of several interesting reasons to support Latin. However it is logistically impossible for Microsoft to provide data for all of these and choosing is also quite difficult. How can we explain to our customers that we chose Latin instead of their native language, and likewise how could we explain to you that we chose another language instead of Latin?
Since we cannot provide cultural data for everyone, we instead made it possible to use the custom cultures so that users can provide data for themselves. I would hope that groups needing Latin would share the Latin culture they create, and that other users would share their data. I wouldn't be surprised if users created a place to exchange custom locales or if a company decided to market packages of Custom Cultures for specific needs.
Microsoft will continue to add and maintain locales, however many of them will remain difficult and we will continue to have limited resources to assign to this problem. Custom Cultures do provide our users with a way to create the data sets that they need, and we hope that people will use them to solve this kind of problem.