As a spectacle wearer I find the eyecup invaluable and now carry a spare. It is a shame that they're not a more secure fit as it is easy to snag them when you take the camera from the bag, if it wasn't for the fact I often use a RA finder I would glue the eyecup on.
I lost one for my F100 long ago in Disneyland Resort (the one in Anaheim, CA). Anyways, I replaced it with one from bocaphoto...and got several under the premise "two is one and one is none." I have not lost any since. I also found that they work with my D700.
I sit here chuckling as the replacement eyepiece I ordered form Adorama arrived yesterday. Timely topic. I had noticed mine coming off when I took the camera in and out of the bag (as somebody else noted above). Suddenly it was gone after shooting out in a field of flowers. I ordered two on the basis I figure I will loose one.
Nikonians is the Smithsonian of Nikon knowledge. If there is a question they can't answer, I want to see the question.
I've been using cheap eyecups that slide on to the square Nikon eyepices for years and slide off just as easily. I typically lost one or two a year. I recently moved to the Hoodman eyecups. They have a catch to keep them in place and work well. It would take quite some effort to knock one off. I don't see myself using anything different in the future. They are the last word in eyecups in my opinion.
I used the magnifier on the D300 for awhile. I decided to stop using it because it necessarily narrows the angle of view a bit, such that I would make exposure mistakes because I had to go out of my way to peer down to the LCD display below the viewfinder.
Other than that it worked fine. I do more manual focusing on my D700 and on that camera I do still use a magnifier, but have the same problem, although on that camera it seems not as bad.
Probably something you have try to see how you adjust to the slight "tunnel vision".
I've never had the stock flat eyecup fall off, after over 70,000 frames and almost 4 years. On my D70 I tried one of the larger rubber eyecups. That came off every time I put the camera in my Lowe Pro Mini-trekker and one day it disappeared.
The larger eyecups are nice to block light but don't stay on the D300 style viewfinders. In that way the D700/D3 style round screw in style works better. I use one over the magnifier on that camera.