In my experience Samba does not cope well with an unstable link so there are good reasons to avoid it wherever possible.

There is also the security issue, by default it could let anyone else connected to the same cell tower as you browse your shares.

Fact is, by making it hard to install Samba (at least as a server) then it means only the people who REALLY know what they are doing will do so. As already mentioned, the module to support connecting to shares is readily available but only in extras-devel I believe as its not something you want kicking around a novices N900. Personally I have not tried SMB shares yet as I suspect it would be a real problem when you forget and roam out of WiFi range, its not something SMB was ever designed to cope with.

Connecting to Samba has always been problematic for me even in Windows, if you are using WiFi and the link suddenly drops it tends to hang the share, or if the network takes slightly longer than expected to come up it fails to reconnect your mapped drives. Quite frankly its just a royal PITA so I can understand why Nokia are discouraging its use.