Here are some rational reasons why consumers might publicly and self-interestedly advocate for products they have already purchased:
1) If the product maintains a good reputation, early adopters will have been seen to have good taste or perception, which is socially valuable.
2) If you have developed expertise in a product that grows in popularity, you have the potential to be a resource to late adopters. Whether that potential is exercised or not, it represents a certain amount of social, and in some cases professional, capital.
3) If your product becomes disfavored, it may become unsupported. This means that you likely have a shorter usable lifetime for the device, fewer support options (official, 3rd party, and community), and fewer future options for peripherals and software. Each of these network-value losses means less personal wealth.
Also, there's some evidence (see e.g. http://en.wikipedia.org/wiki/Endowment_effect) that people often overvalue past purchases even beyond the level that rational assessment would indicate.