The various cmdlets included in the PowerShell snapin, were built around some use cases we came up with. The LTMPoolMemberItem is native to the snapin and we never really documented all those as they were really originally built as a set of sample code. PowerShell allows you to dynamically query the structures for types but that's not always the easiest way to go.
If that cmdlet isn't really working for you, you can always use the (Get-F5.iControl) cmdlet to return an iControl.Interfaces object containing all of our core iControl interfaces defined in our iControl API.
https://devcentral.f5.com/wiki/iControl.APIReference.ashx
You could then do something like this
(Get-F5.iControl).LocalLBPool.get_member(@("pool1"))
which is documented in the API here:
https://devcentral.f5.com/wiki/iControl.LocalLB__Pool.ashx
Hope this helps...
-Joe