SigmaI admit that I haven't checked if flashing a bunch of people then switching weapons slows the drain rate, but I don't think it does anymore. At most, it might reset multiple flashes to the base drain rate, but I doubt it. Either way, it's not very practical because you almost always need to be ubered yourself while flashing, and the weapon switch + switch back delay is more than enough time for someone to react and shoot you. In the past (up until early 2008 iirc) your uber would just not drain at all if you switched off your medigun, so you could keep someone ubered for like 30s, but the medic wouldn't be ubered because you lose uber instantly and don't flash when you switch weapons (and thus it was rarely useful).
I think you guys might be misunderstanding what I said. The rapid weapon switches don't slow down the uber drain rate, it stops it extremely briefly. And yes, back in early 2008, you could store whatever uber you had left over by switching weapons and then bringing out the medigun whenever you wanted.
Again, I agree that it's not practical almost ever. I found it useful only in situations where I was forced to uber, then the guy who forced me to uber ends up dying instantly and perhaps the rest of their team is dead or just spawned. There's very few situations where it could be used, and it probably didn't make a difference even when it could be used. It was just a fun fact though I guess, haha.