When upgrading cars, it's possible to purchase wheel spikes that add a spinning metal blade to your car's tires, ostensibly to pop other cars' tires and to injure pedestrians—so far, though, I haven't actually seen any tires popped as a result of this upgrade. Does this even do more damage than simply ramming such targets off the road? What's the benefit of this upgrade?
Wheel spikes do indeed pop tires. They extend from your car's tires gradually as you start to accelerate, so they won't be at full tire-popping potential until a few seconds after you start moving. While there are only a few points at which this can be an advantage, the upgrade's pretty cheap and you might want to take it on at least one car anyhow.
For one thing, popping tires is not considered an additional hostile act, and in fact I think it might not be considered hostile at all in certain cases - I've popped cop tires before without getting a police wanted star, and I've popped other tires in full view of police without upsetting them, but I've not tested it exhaustively. Careful not to hit the enemy vehicle hard, lest you upset them.
This means that it won't increase your wanted level, but it can get pursuers off your back. Dave's right to point out that the AI rarely has trouble with popped tires (it seems to be able to set speeds with vehicles that are beyond the top speed they can normally travel) but they do have trouble with popped tires and tight turns. The Luchadores especially fall victim to this with their penchant for large trucks.
I don't think the pedestrian injuring features are all that useful, more often than not it will cause you trouble rather than solve anything.
The other issue I have with car upgrades in general is that most of the missions and other diversions require you to drive a particular car, and getting your own car out of a garage is usually rather painful. I upgraded several sports cars, thinking I'd have a use for them, but by and large I just drove whatever was closest to me while I was on foot.
Yes, they will pop tires and injure pedestrians, but you have to drive extremely close to other cars to make this happen, and honestly, if you're doing that, you might as well just sideswipe them in the first place.
Keeping in mind that blowing a tire doesn't affect a car's handling in the Saints Row series much, especially not for AI drivers, wheel spikes don't accomplish anything worthwhile. If you have extra money (and you will after you complete the main storyline or a lot of the activities), then sure, add them for the occasional slash.