André,
Well, according to Veeam that's not true for thick disks on ESXi. (With thin disks you'll get fragmentation over time anyway.)
"With ESXi, preallocation is only possible for thin disks, and in this
case we do preallocate. For thick disks and any other files, it is
simply not possible at the moment with VMware APIs."
The big question is why the API allows pre-allocation on ESX but not on ESXi.