Each to his own, but looking at the ETKA parts catalogue the current A3 uses blades that are 600mm and 484mm.
Having looked on a couple of web-sites selling Bosch Aero blades they only offer standard size blades of 600mm and 475mm. Not a lot in size but could allow the water to run up the screen between the blades, particularly when they stop. The price for these blades was £32.95 for the pair plus £2.50 postage, so a total saving of around £5.00 for not the correct size blades. The blades fitted by Audi may well be manufactured by Bosch, but to Audi's specification and size. The blades listed on the web-sites were for all A3s since 2005, but the method of fixing the blade to the arm has changed at least a couple of times since then.
Is it worth it? If your car is still under warranty and you had a problem with say the wiper motor, could a dealer argue that non-standard parts had been fitted?
I have used non Audi blades in the past and they have not been as good as the OEM ones. Having had them on the car for a while I changed back to OEM ones which did a better job.
Personally, my A3 is covered by a 3 year maintenance agreement which I negotiated to what I consider a reasonable price and it covers things like the blades anyway.
It strikes me a bit as 'spoiling the ship for a ha'peth of tar'