"it's really not hard to run Ethernet cables around old houses and flats" - Beg to differ.
Yes it is. I've a ~100 year old house with mostly solid walls and ground floor, and if I wanted to hide Cat 5 cable, I would either be digging into the lath and plaster of the upper story walls (no easy cavity in these) or solid walls (no plaster board anywhere), or pulling up skirting boards that have not been touched since the house was last re-wired (or in some cases, since the house was built).
Because of the size of the house, I would probably want a switch on each floor, limiting the number of floor-to-floor cables (it's a 3 story house), which would need me to find a location with power where I could install each switch. It would be much more of a commercial PDS rather than just a Cat 5 cable or two.
Also, I cannot use wireless throughout because of the solid walls that prevent wireless signals reaching the whole of the house.
It's MUCH MUCH MUCH easier (a few minutes compared with months of work) to plug two homeplug devices into the mains sockets where they are needed. And I can move them whenever I want, and I don't suffer the wrath of the Wife.
My house is a really switched on one with more computers than people, with 4 tower systems away from the room containing the ADSL router, Firewall, NAS storage, printers, and Wireless Router. I use homeplug for the towers, and wireless for the laptops (and the MacMini and Wii which have wireless built in and are in range of the wireless router). I also use a homeplug for the Xbox360, because I already had a spare homeplug, and did not want to shell out for the inflated price of the Microsoft Wireless adapter.
I have a mixture of 85Mb/S and 14Mb/S devices, but I find that two 85Mb/S devices talking together are easily faster than 802.11g (with good signal) for transferring large files around, but obviously slower than directly connecting to the 100Mb/S switch.
So, yes, it has solved my problems and yes they were real. My only concern is how rapidly I am loosing the older homeplug devices due to failure, and whether any of my neighbours are SW or HAM radio operators.
Now. Go back to your modern shoe-box house and think again.