Maybe that wasn't the best example, alright lets try this:
In a normal competitive game, if swampert supposedly got sap sipper, would that really matter in the long run? It still can't take a bunch of hits for too long, so wouldn't it be easier just to outspeed all of the pokes and just eliminate them offensively while still being tanky enough to take some hits along the way? Most special attackers and some very powerful physical attackers that would destroy swampert like latios, keldeo, shaymin-s, crawdaunt, mega-medicham, who don't necessarily have to use grass moves to bring down normal swampert, would now find it hard to kill a very fast, offensive, M-swampert. Swift swim would work 95% of the time and is much more consistent, when sap sipper is situational at maybe 5% of all games at best.
Honestly, what do you beat with sap sipper? Victini can still hit you with Psychic, which will still hurt. trevenant can still curse+will-o-wisp you to death. Tangrowth? I guess I could give you that but sleep powder would cripple it for the time being.
...I just don't see sap sipper being that useful other than being an immunity. An immunity is always nice, but when it doesn't address core problems, then there is no point.
At least goodra has enough bulk to survive other coverage moves from pokes and hit back with its diverse moveset that covers the majority of types with some support moves like infestation and such. Sap sipper makes sense on goodra in this case.
edit: if you only replied to that one statement and nothing else, and you understood everything else, then ignore this comment.