I have toyed with the idea of fitting a remote reservoir on my Europa simply to raise the reservoir and provide improved head pressure for maintenance, bleeding, etc. As it is now, the height of the OEM reservoir can't be much higher then the bleeders on the front calipers. I will eventually tackle a complete brake service project (all new lines, other mods, etc) and considering the remote reservoir for that reason alone.
It is a strange one, the remote reservoir was standard on the UK single circuit brakes and honestly there's no great effort to fit a remote to a tandem m/cyl. I can only think that when Lotus moved to dual circuits for the Federal cars they decided it was cheaper to leave the tandem cylinder as it came and save on the production line costs for a remote + pipework.
Having said that, the single circuit m/cyl is much easier to fit a remote to, the reservoir connection is threaded so it's easy to fit a feed pipe.
When I was researching braking systems I found several references to the advantage of having a head of fluid above the tops of the calipers/drums. So yep, if you're refreshing the whole system I'd say it's a good idea, plus of course it's much easier to fill and monitor.
Brian