Its just a measurement. It can and will change over time and while its a good relative indicator of how well you can perform over time, its not a goal itself. FTP is a process measure, not an outcome measure. It should be used as a means to tell you your training is working, and to guide your training. Its relatively easy to get a better FTP in a field test than what you really are capable of performing and thats even easier to do if you do your test on an indoor trainer. The worst part about it is that using an FTP that is too high (or too low) to set your training ranges will probably reduce your rate of improvement.
Finally, no, you shouldn’t have it on a T-shirt or worse on your tag-line on a blog post or an email – that just makes you a dick.