I'll say it again:
#1: It's the only way to get any sort of payout directly from those buildings. Buildings that don't pay out but can be vandalized should be vandalized for this reason. (BTW, all buildings that can be vandalized do not pay out)
#2: Cleaning graffiti up is a good way to raise your Righteousness rating on the conform-o-meter. And until you're high enough level to get the training wall decoration, it's pretty much the only way to do so, other than catching Sideshow Bob in your town. But Bob is random, and taps from friends are not. Before I could buy training walls, I literally lost all my righteousness stars every time I leveled up (I don't know why, some random glitch) and it was very frustrating to me to have people visit and not give me any graffiti to clean. I always had to ask someone I knew to please go and vandalize my town. For this reason, I always make it a point to go after the spray cans, because I don't know who might be needing it. And even if you don't necessarily need it for Righteousness, there's still point #1 with the money and xp. I mean, I know it's not a lot, but it's something.
#3: It cannot mess up anyone's collection schedule. It's not always just house farms that are going to be on a schedule. There are some things on the edge of my town I know to go collect because something else is ready to go. Pretty much all of my same-time buildings are synced. One of the things I liked most about halloween was that this was rarely affected. Now, I realize that not everyone is going to be as satisfied by same-time collections as me, but I also doubt I'm the only one.
#4: You can always do it. Even if I've just tapped every building in my town, you can come in and help me out by vandalizing buildings.
(When I badly needed graffiti, I would just keep my tablet open, making sure I had very few things to tap on that weren't graffiti. It drove me up the wall when someone would tap on the Simpson house instead!)
I'd like a couple more buildings to vandalize. Having a total of four seems odd, since friends get three actions in a town. There should be at least six. (I wish the cooling tower could be vandalized, personally. The power plant is another building that only pays off if someone is doing a job there.)
There are people who claim that getting tagged lowers their rating, but I can't imagine that being a huge problem unless you don't log on for a week. Also, as I said, the Righteousness will drop off just absolutely randomly, and then you really need the graffiti to clean up.
All that said, the best thing you can do for your friends right now is see if they have a squidport tile, news broadcast, crop, or stage performance you can tap on. I still like getting vandalized, but the best thing ever is someone tapping my news station and squidport. (and farm, but the news station pays out more :wink: )