For more precise and detailed information, I think you may search at www.baidu.com for 儿化音，or you may search and download (for free ;) books such as 普通话教程, 普通话培训. These books are intended for Chinese people who're learning Putonghua, but will help foreigners with R sound as well, I suppose. It's true that the major Chinese population ( including a great portion who speak northern dialects) do not have the R sound in their own languages and therefore they do not pronounce the R sound even when speaking Putonghua. The reasons supposedly are 1) habits 2) they donot know where should be and where should not be an R sound in Putonghua 3) Some people have difficulties in making this sound, especially for the more southern people, because there is no such a or similar sound in their dialects. But the R sound does not come disponsible in Putonghua. That's a must for learning standard Putonghua. (For more information, please consult the above reference). < But, as you know, lots of Chinese people do not prononce this sound very well when they speak Putonghua, so it does not matter so much actually, on the other hand ;)> Chinese linguists has long been trying to make a R-sound word list as national standard, but not yet achieve it. The problem is, although R-sound is necessary in many words, it's otherwise optional for some words: it's depending on the individual habit when its disappearence does not change the meaning of the word. I think you would not have difficulties in making the R sound, for you come from Spain, and there are similar sounds in your languages. In fact the Putonghua (Vowl)-R is pretty the same as that in the American English (GA). eg paRt, letteR, giRl. What's different is that when we pronounce Putonghua R we do not curl up the tongue edges.

视具体情况而定，能带“儿”的地方一直能带“儿”，不能带“儿”的地方永远不能带“儿”。

Yes, it can change the meaning of some words by adding "儿'. For example,"头“means head,"头儿”means leader. Besides, it can distinguish the the characteristic or property of a certain word, “画”is a verb or noun,"画儿“ is a "noun". Most of the time,if "儿” is added, it shows relaxation，warm feeling or love for something.

I'd say the majority of "r" use is northern dialectal speech. Southerners don't tend to use it much, even where it's prescribed they should. By dialect, of course, I mean dialects of Putonghua, not the native tongues of various areas. Beijing's native tongue, for instance, is pretty much exactly putonghua save for a scattered word here or there, but I've seen Beijingers use the r sound a bit less when they try to adjust their speech into national standard. Southerners use it when it's an issue of meaning, but even then, it doesn't seem to be. A very popular sound.