"This is actually one of the ironic casualties of the cloud. In the old days you bought your packaged software and it was pretty much guaranteed to work as long as you didn't change your setup (OS, hardware, etc)."
And this led to a lot of other problems, for example, you couldn't keep up with new technological innovations because you needed older systems for some programs.
It's an interesting question, which practice is better.
I think the best approach is to make sure whatever service you decide to use has good import/export features.
A boss at a job I worked at a few years ago still keeps his contacts in an ancient version of Lotus organizer without VCard export support. As a result he's pretty much locked into it unless he ponies up $100 for the latest version of the software (released in 2003) which has VCard export. He's too cheap to do that so he'll have to stick with it as long as he is still on a version of Windows that supports 16bit software.
And this led to a lot of other problems, for example, you couldn't keep up with new technological innovations because you needed older systems for some programs.
It's an interesting question, which practice is better.