You do, because it's always easier to compute proper and exact numbers rather than using an approximate HP bar.
If I calculated beforehand that 15K rDPS are needed in order to pass DPS check #1, by having a quick look at the DPS meter, I can tell how my raid is doing. If we are on time or not. If I see 18K, then we're good. If I see 15K, then we're just there and I would give the instruction to raise DPS a little bit etc.
As for your Garuda example, again, if some players are retarded and don't play properly just because they want to be first at DPS, the issue are players themselves, not the tool. Parsers, meters, add-ons, all of these are fine when used properly. We shouldn't restrict any of these tools just because some persons misuse them.
If you wipe your raid because you wanted to top DPS you're just an idiot. Having no addon won't change that, you'll still remain an idiot. However pushing DPS because rDPS is at 14.5K and 15K are needed to pass the check makes you useful to your raid. Individual risks are better than a raid wipe...