I thought I'd share what I finished with. I doubt the drive count is the most elegant solution but it works for me. I wanted to loop through the filers and get a count of all the spares on each system. Here is the code;
$xmldata = [xml](get-content c:\scripts\data_files\netapps.xml)
$filers = $xmldata.netapps.filer | select name
foreach ($a in $filers)
{
trap{"Error connection to " + $a.NAME}
$controller = Connect-NaController $a.NAME
Write-Host $controller, (Get-NaDisk -controller $controller | Where-Object {$_.status -eq 'spare'} | Measure-Object).count
}