It does make sense if you have more than one car or have higher mileages. Like I say think of how much diesel / petrol you might put in a month and you never bat an eyelid.
However paying for oil that you might use for a year no one wants spent more than £20...
Does seem a little backward.
If a car is your body. The oil is the blood. Don't put the wrong stuff in, Put the right amount in, don't run out and buy the best you can.
All of this can easily be done for £25 if you do it yourself.
Even with limited tools. A hex bolt for the sump plug and ractchet, a 5 litre drain tray and a screw driver you can do it in 30mins. You don't really even need a set of ramps or a jack.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts