Hi Rogelio,
View identifiers must be valid C identifiers for a variety of reasons. Basically, view identifiers are expressions in PixInsight. For example, they can be used in the command-line interface, and also in PixelMath expressions. In the future, the PixelMath syntax will be extended to turn it into a more sophisticated programming language, which will be available on the whole platform, not only within the PixelMath tool (most likely this won't happen until PI 2.0). The PixelMath language will also be used in the LayeredComposition process.
If we allowed free view identifiers, then we'd be compromising future development of many critical tools and features in PixInsight. On the other hand, that would require inelegant changes in the PixelMath syntax.