ROLES IN TIME AND SPACE. “Greater than 95% of human multi-exon genes express multiple splice isoforms. Furthermore, there is evidence for alternatively splicing events that are often differentially regulated across tissues and during development, as well as among individuals and populations, suggesting that individual isoforms may serve specific spatial or temporal roles.”
http://bitesizebio.com/10148/what-is-alternative-splicing-and-why-is-it-important/