Mol, I think a first aid kit, albeit basic, is quite a thoughtful addition at the property - some groups, no doubt, are so busy focussing on the clothes they need when they pack, and sun protection etc etc, toys/baby equipment blah blah, that their thought processes go no further. Not everyone has a checklist - although I had one I printed off before each holiday when our kid was small. Since then I haven't much bothered, and have been caught out a couple of times! I'd be willing to take a bet that failure to take basic first aid stuff etc on hols is something most of us have done at some point on hols.
The presence of first aid kit at your property confirms you to be a caring owner, and that may stick in someone's mind when they plan next year's holiday, hopefully!