You removed the caps "just because" or was it vibrating unusually badly? If the mounts are good, the bike will have what could best be described as soft vibration. If there's any hard hammering or shaking that you feel, the front mounts are probably bad.
They're just holding the engine with the rubber in shear. The mounting holes are offset to the top of the mount (not concentric) so that when they're loaded with the weight of the engine, the screws more or less end up in the center.
If the rubber fails, the screws are designed to let the engine drop down onto the spacer collars but, still keep everything where it belongs. It'll vibrate like hell but, nothing will fall off.
So if you see a gap between the bottom of the spacer collar (inboard of the socket head cap screws) and the surrounding metal housing, they're still hanging on. They might still be getting old though, and sagging more than they should.
As others have already suggested: they're a wear item. They go bad. Install them carefully (don't introduce any twist into them as you tighten) and they should last a long time.
And before some Harley dweeb tells you their bikes are different, Harley adopted very similar mounts, with all the same pros and cons, quite a few years ago:
