Speaking on a purely personal basis, as someone who used to be able to afford to rent several sims from LL and who can no longer afford to do so in the current economic climate (mostly down the UK government's taxation) , the who is online has been invaluable to me. I had a need to ban a number of residents from my sims, some people hold grudges, most I have muted but that does not stop visits, from time to time, from previous pests. Knowing if they are online gives at least a 'forewarned is forearmed' sense of security (motto of the Royal Observers Corps).
From a purely selfish and personal point of view, there may be times when I am AFK and in busy mode and someone will contact me. If I'm at work I could be away for 12 hours, if I know they are offline when I am on then I am not upsetting anyone by them thinking I am ignoring them, same if they think I am offline, works both ways.
The who is online is, I believe, an innoccous feature that does a lot of good and zero harm. I would like to add my name to those who would like the service retained please.