Chinese culture is converging...I think that is true...but the culture converging is not only happen in China,but also spread all over the word, just like a lot of people speak english and wear T-shirt and jean, no matter where he/she comes from. Globalization makes the whole word becoming more and more similar.
Well...if you know about the chinese history, it's not hard to find that from thousands of years, the king of each dynasty they do a lot of work to make the culture converge, and it's turned out very benefit for dominion over such a vast area.Like the first Emperpor of Qin (259-210B.C), he unified the characters,so even we can't understand each other's dialect sometimes, we can still appreciate one another's literatures and so on. Also that's a very important reason for why in four countries with an ancient civilization, China is the only one who's culture never breake up.
As for the mandarin...that's really makes our life more convenient,but it's also harmful for the local culture if we never study the dialect...I feel lucky I never lose this 'special language', I talk my family with dialect, but when I go to school I talk with my teachers and classmates with Mandarin. I think most of Chinese have this habit since he/she was a child. and also I don't think it's a hard thing to speak perfect Mandarin if you practice and imitate a lot. :)
Hmm...you know, an increasing number of people realized it's very important to protect the local culture and to keep the culture diversity. I think China do a lot of work for this...at least in music field, I know the national folk music is becoming an focus in recent years. Many musicians going to the mountain villiage to record these folk songs which just pass by mouth and singing with dialect,that's pretty meaningful . :)