I think the bobbing up and down from the forks gives people the impression your flashing them to pull out.
Run with lights without a battery and they'll really think you're flashing lights at them, especially at intersections.
I kinda agree and disagree but this is relative to what charging system you have.
A standard RM19/21 will be a different kettle of fish I think.
My only experience with running with only the big blue cap was a '74 T150 with the RM21 system. I used the old sealed-beam car headlight at the time, it dimmed at idle. The indicators only flashed at over 2000rpm, and hitting the dual horns at under 3000rpm would kill the engine. At least it would keep up with the brake light.
I have been stopped by the police for "flashing" my headlight at them at intersections when revving the engine.
The cap will save from trouble starting when your short ride did not charge the battery though. I had one in concert with a battery on my A65. Once removed I had some dead battery events on short trips. That bike now has an RM24, no problems.
In short, the capacitor is fine for competition, or if you can get away with minimal electrics on public roads, but for a fully equipped street motorcycle it simply is not adequate.
BTW: If you decide to use the cap in concert with a battery put the cap on the switched lead. A leaking cap, directly across your battery, can wreck your day.