I think what you are on about are ADSL filters; one of these is requied for each appliance connected to your phoneline for adsl broadband. That includes your phone, extention phone, fax, house alarm (if conected to an alarn recieving centre) and answerphone etc.
These are required to filter the two signals, the phone siganal and the adsl signal, both are on different frequencies and the filter prevents them from interfering with each other.
I use wired, woreless and Homeplugs to connect my 2 PCs and a PS2.
If the encrytion is set up properly on the wireless router and the SSID broadcast it turned off then the network is very secure.
What you may be thinking is that Homeplug is secure because nobody can 'piggyback' onto your network but this is possible if a neighbour is on the same electrical phase AND is close enough (electrically speaking) AND can get through your firewall. As you can see, even with no encrytion on the Homeplugs it's very unlikely that this could happen.
If you don't want a router then you can connect the Laptop to the Desktop via 2 Homeplug units and use Internet Connection Sharing (ICS) but it will be much more convenient to cannect the lappy wirelessly to a wireless modem/router..