This is an interesting topic, and here are my thoughts on this:

Naturally, the keboard makers want us to buy a new one every year. They also want us to keep our old one, so as not to reduce the size of their potential market. To do that, they introduce instruments with new features, some useful, some simply gimmiky. The market leaders also seem to take some of the functionality away (so that you don't feel that you have the "ultimate" machine, and look for the next one when it comes out. In a way, this is, like you said, a seduction.

Some people simply need to have the latest gear. They will buy whatever new model comes out, simply to have the knowledge that they are in the forefront. This works really well for those of us who have unlimited budgets, and this phenomenon is not limited to keyboards - I know people who trade their home computer to the latest model every 6 months, simply to be able to brag that they have the fastest processor. Most of them do not need the power of 1GHz Pentium, because their old 600 MHz machine was fast enough, but, hey, this is a free country, and one may spend their money the way he wants (as long as it is legal). I don't quite belong to this category: when I come into the music store and see a new machine, part of me wants to buy it, but the other part of me (usually my wallet) stops me, and starts questioning the real improvements of the instrument.

The musical instruments, though, are somewhat different from computers, as their mprovements, far from almost meaningless claims of a microprocessor speed, can be heard and experienced, so it is somewhat easier to justify an upgrade. As a (part-time) performing musician, I would know it is time to upgrade if a)I find that the instrument makes me sound a lot better, b) it has new useful features/allows for greater portability, or c) the instrument is much more user-friendly than my current one. Another factor would be I had to keep abreast of the competition, with a newer machine with some features, albeit useless, which the audience loves (such as easy accessible applaudicements sounds, etc.).

Ultimately, this comes from the fact that not being a true virtuoso in any one istrument, nor a singer, I have to rely on the computer inside the instrument to help me show my talents (however little I have). If I was playing piano, I had a 90-year-old instrument and did not feel at all interested in exchanging it. Similarly, I had a 30-year-old clarinet, which made me sound every bit as good as would the newer ones. I knew that the only way to sound better with those nstruments was to practice more time daily and improve my technique. With the auto-accompaniment instrument, however, I can sound almost like a 10 piece band while practicing only once a week (with four kids and a day job, that's all I have time for). If I can buy a new instrument which makes me sound like a much better 15-piece band, and makes it look lime my technique has improved, it may well be worth it. However, playing on stage, my criteria for user-friendliness and worthwhile features are stricter than those of people who mostly play at home. Hence, I do not usually rush out to order a new instrument whenever it is announced.