actually some lichens are rare (though I probably couldn't recognize those)...
...but my understanding is the "crunchy" lichen which gets broken up when you step on it, is sort of like the brooms in the Sorceror's Apprentice, you're just spreading around lichen spores. The non-crunchy lichen like map lichen (see upper left of this
diapensia photo) is attached well enough to the rocks that you'd practically have to scrape it off with a chisel to damage it.
the effects of foot traffic on alpine ecology are a complicated issue (I'm not an expert here but this is just some food for thought)... if you just stepped onto a blade of grass (or sedges/rushes which sort of look the same) & pressed it against smooth rock or small soil particles, I'm guessing that probably wouldn't do any significant damage. If your boots scrape the grass against rock/soil or tear it, however, you could damage the individual blade; how much stress that puts on the plant itself would be unclear. Or you could step on the lower part of the grass near the base which is probably more prone to damage.
As far as individual plants go, most are common enough that an occasional loss of individual plants is not an issue. Sometimes there are very small colonies of plants (<1 sq m) where loss of an individual can make a difference -- dwarf cinquefoil nearly got decimated because of overuse on the Crawford Path, and I seem to remember hearing about several species which were extricated from the heavily-traveled Franconia Ridge. Other times the detrimental effect of foot traffic isn't damage to a particular plant but rather erosion (go to the summit of North Carter sometime) or initiation of further weather damage (if you damage one section of a diapensia cushion, it creates a spot where wind/weather can enter and cause further damage long after you're gone) or allow competition from other plants which do better in disturbed soil (go to the summit of Hale, see the fireweed & mint).
Do what you're comfortable with, but it'd be nice if we all understood the impact we have...