Is there any point to opting out of interest-based ads?

Yes, there is a point.

To be clear, "opting out" merely stops the targeting of ads. It does not eliminate ads, or reduce them in number, or vastly reduce the more insidious Tracking aspect of ads. In a broader context than just Chrome, it varies by platform and ad system, but some ad networks will not collect as much information knowing that they can't target from it. So there may be a marginal privacy advantage from that.

But a very real reason to "opt out" (assuming you don't block trackers outright, which is the vastly preferred privacy solution) is: it may send a signal to the ad networks that you don't consider cross-site / cross-device targeting to be a fair trade-off for viewing the affected pages. In addition, it may even reduce the revenue to the ad networks since they can't sell presumably higher-cost more-targeted ads onto your page views. The signal plus the possibility of reduced revenue may in the long run contribute to a healthier (for users and user privacy) ad ecosystem.

But keep in mind that Blocking is the only way to get a large immediate privacy advantage over ubiquitous Tracking.

The only thing I can think of at the moment is: some companies use these ads networks to remind you of things you have lately browsed in their site to push you forward to buy them. So imagine you are looking for some sexy underwear for your wife in your device and later at your work they show up while showing some sites to your customer (fake situation but possible). So using those services may prevent those situations. I don't feel it improves your privacy in any way.

Those extensions and similar ones do exactly what it claims to do (and they are opensource): reduce targeted ads from companies tracking you online and as you concerned about the privacy issues, the IBA Opt-out extension you linked to for example states:

Stops interest-based ads on some of the websites that partner with Google

In the Protect My Choices extension you linked to, Google Overvie statement says:

Protect My Choices applies to interest-based advertising by participating companies. Once you install this extension, you will still receive online advertising from participating companies, and the Web sites you visit may still collect information for other purposes

This means those Google partners (companies) still pay to have their ads displayed there for users so there will still be targeted ads on areas outside Google's control. So you can see your privacy is the last concern of Google which is rather making everything to keep advertisers happy.