I somewhat agree, though I'm of two minds about it.

On the one hand, I think that it's true that highly publicized cyclist deaths and public mourning of them probably does encourage some people not to try riding themselves. We have to remember that most non-cyclists are already predisposed to believe that cycling is an extremely risky activity, on par with "extreme sports" or something, because they often only consider the "passive safety" that their vehicles provide them rather than the "active safety" that comes from being alert and avoiding crashes. We had a highly publicized cyclist death here about a year ago, where a (right-way) sidewalk rider was killed by a right-turning driver as he left the sidewalk to cross a side street. Many non-cycling friends and acquaintances I have are STILL bringing this incident up as some sort of "proof" that cycling is inherently extremely dangerous and telling me I'm going to be killed unless I stop. I've tried to explain about how yes, sidewalk riding can be quite dangerous, especially if you pass large trucks with poor side visibility on the right while they are turning across your path and signalling their turn in advance. I also explain that this is why I almost always ride out in the traffic lane in that and similar congested areas: to minimize intersection conflicts and maximize my visibility. The non-cyclists just seem more horrified by that explanation, 9 times out of 10, because they aren't able to wrap their heads around the idea that riding in the street may be safer. I think they're usually thinking that the cyclist was actually doing everything right by riding on the sidewalk until I tell them this, which is why they're viewing it as some unavoidable tragedy. So I think it's true that media publicity of cyclist deaths does scare some people off in a way that publicity about car crashes doesn't: you have to add ignorance into the mix, because it's an activity that is more foreign to the experience of most people.

On the other hand, I don't think these rides of silence are really necessarily about "advocacy". Instead, I think they're more about mourning lost friends (or at least, friends in spirit) and catharsis for the people who participate in them. So I think they're a completely reasonable thing for cyclists to do for themselves, if they feel it's an appropriate way to honor the dead. I just don't think it should be viewed as an advocacy action.