I have been using external binaries since Poser 9 SR2 and never had a problem anymore. There were problems prior to P9 SR2 but they have been fixed.

Why use it?

Filesize is smaller (yes, even with the PMD). But the primary reason I use it is because file loading and especially file saving is a LOT faster. If I have a very large scene (800MB without external binaries) it now saves in 3-4 seconds instead of almost a minute if saved without external binaries. So I do save a lot more during a session

Since I started using it full time I have created hundreds of scenes with all the different DAZ figures and other 3rd party figures. So I think it is very reliable, but that is a decision you have to make yourself