In the past spitting was quite widespread, some pubs would have spittoons into which people were encouraged to spit rather than on the floor. As society became aware of the connection between the desease TB and spitting, then it became less acceptable: buses, public buildings, swimming baths would have signs saying no spitting, in France "defense de crache". School kids are injected to stop them getting TB and the killer disease has almost gone, certainly it has left the public consciousness. However with not all children being injected (for whatever reason) TB is on the rise and the growing acceptability of spitting represents a potential health issue in years to come.
I have on occasions had to spit; fly in the mouth, stung on the lip. But, other than in an emergency I think it is unnecessary, unpleasant for others and ultimately a selfish and anti-social action. If I am unwell, or covering a long distance I carry tissues. Again in emergencies I have also blown my nose on my shirt (but that is my business and does not affect others).
However this is trivial when we consider the number of runners who urinate (and worse) inappropriately and willy-nilly. I have seen large numbers using public green space next to pavements in a City Centre, while they were walking to the start line, having left the portaloos a few yards behind at the pre race Assembly point. And that was only a Half Marathon.
These unnecessary behaviours will give runners a bad name and in some cases gives local communities reason to object to future events.