+1. Watered down beer sucks.
Sorry to revive a dead thread, but I thought I'd add my $0.02 in case anyone reads this later. I've topped off my beers at all the different stages (during primary, during secondary, or at bottling) at one time or another, and never had any ill-effects that I could tell...
I don't do it very often, and when I do it's not for the usual reasons that many state (such as winding up with too high FG ...I generally consider that event more of a bonus than a failure.
. Fact is though, if one is careful, the beer can be topped up at practically any stage from pre fermentation, to kegging, or (more rarely) at serving time.
Sometimes, if I want to have a beer and not be concerned about getting buzzed, I will occasionally make an "L.A." beer by pouring a robustly flavored (but not necessarily strong) beer such as a porter, and cutting it with a some plain, cold seltzer, right in the glass.
Sounds odd, I know, but the result is refreshing, lower in alcohol, still packed with flavor, and it allows me to have a pint in the middle of the day and still get my work done
...without getting my mixed all words up.