When Telstra announced it would offer visual voicemail on iPhones, we ran through an analysis of whether the service was worth paying for. Here's one specific scenario we didn't consider: retrieving voicemail when overseas.
Australian Business Traveller has analysed the data usage for Visual Voicemail, and concludes that it comes out at about 85Kb for a 30 second message. As such, that can make visual voicemail cheaper to download than listening to your voicemail while paying global roaming rates, especially in countries like China where those rates are often very high.
The analysis makes the worthwhile point that roaming is generally an expensive option, and that you should use Wi-Fi hotspots whenever possible rather than relying on roaming data. My personal travel strategy is to not enable data roaming in the first place, change my voicemail greeting to make it clear that I'll respond more quickly by email than by phone, and stick to using Wi-Fi or prepaid dongle options.
But if you are a regular overseas traveller and work demands that you have voicemail access, it's another potential argument in favour of the $5 a month fee. (In that context, it also seems like a small amount compared to what you're already paying in airfares!)
How the iPhone's 'visual voicemail' can save money when you're travelling overseas [Australian Business Traveller]