These retro computers are fun - and that's the point
Yes, these retro computers aren't up to much compared to a good PC and a good graphics card etc, but they're fun; simple, nostalgic fun.
Also it's nice to have a computer that doesn't need an anti-virus or some complex OS to boot it. Yes, I know the Pi is almost that, but my The C64 and the Next (oh so tempting) behave in a different, and simpler, way. If you're interested in just playing a few games, get an emulator, but for the whole physical experience a solid machine you can touch (and even "ooh" over - oh, hang on, that sounds rude and even... um, ewh!) and appreciate (like an LP compared to a download music track) knocks emulation into a cocked hat.
Plus, for the average IT worker, £300 isn't much - it's a round of drinks, a filling up of the Porsche's tank, a weekend away for drinking and sex, or even... Well, hopefully you get my humorous point :-)