Ooops, I think we are using the word "growable" to mean two different things.
Any VMware virtual disk is "growable" in the sense that a human being can statically resize it: using vmware-vdiskmanager (on Fusion and other Workstation-family products) or vmkfstools (on ESX Server). Of course, that's often not the end of the story: you usually have to resize the partition inside the virtual disk (unless it was in use by Windows as what it calls a "dynamic disk"), and, if it was bootable, fix its bootloader so that it correctly points at the boot image once again. Dr Dave's advice about building virtual machines with two virtual disks, one for the system and one for data, is extremely wise, no matter what kind of virtual disks you have.
But the choice that bigrock0 asked about is different, and completely independent: should a virtual disk's capacity be pre-allocated, or should disk space be allocated as it is necessary? Maybe a better term for the latter kind, instead of "growable," might be "dynamically growing."
Dynamically-growing virtual disks trade away some disk I/O speed in favor of not having to commit all your disk space up front. Statically pre-allocated virtual disks offer slightly better and more predictable disk I/O, at the cost of a bigger up-front investment of disk space.
