I would do this if I was going to do a emergeny overflow.. ie. just in case a water change gets out of hand..LOL
or a automated waterchange system..
I would do a sump if I was going for a fish only tank though.. you know the wet/dry CO2
problem.. Ill probably be injecting CO2
in the big tank.. I know I could add water to the system and hide some heaters and such but its a little too much work for the added benefit..