If they're invisible, you can see them holding potions or armor I believe.
Potions are a part of PVP. It's very simple. Pots can be thrown and do damage to you. How do you counter this? Dodge them.
If you're going to complain about it even though there is a CLEAR strategy to countering them, then would you agree that we should get rid of bow and arrows? I mean, they can be SO far away from me, what do I do?! I can't even have a simple PVP match with anyone without getting sniped from far away with people with bows. And axes and swords! I can't even go up to people and attack them with my fists without them quickly killing me with their weapons!
There are advantages and disadvantages to all PVP forms. Invisibility is not limited to potions, it can be applied to all forms.
Potion advantages:
-bursty damage
-range
-spammable
Potion disadvantages:
-dodge able
-nerfed to have a slower spam rate
-need a lot constantly to maintain spammability
-only effective in long range. If someone catches you, you're screwed.
Rather than focusing on the advantages of pots, look at the disadvantages and use them. There's a reason people train MCMMO sword and axes instead of relying only on potions.