The way Illusion spells are treated in RM was a revelation for me:
There are illusionary monsters with the possibility to harm you, but your own illusions
are immaterial. RM put it the right way !
With illusionary bridges or swords I always imagine that they also do need a
"Sight Mirage" (or what it is called).
It would be more likely you make a wrong step if you don't see the bridge,
even if it is even. Or you need more concentration.
Or imagine you wield an invisible sword, you have to be used to it, to swing it
the right way (with the edge not with the flat side).
And remember for each strike the illusionary sword or shield do need one "Touch".
...
It would not be a good thing if the party were crossing a chasm when somebody stumbled (removing the touch), or if a flock of birds smacked into it (if the illusion was not visible). Oopsie....
Or if someone get a clue a simple arrow could remove one "Touch".