The objective of pink wards is to scout information of where your opponents are at which is especially important after taking tier 1 towers. This information can be used multiple of ways. You can pick off enemies that are separated, isolate tier 2 towers, avoid ganks knowing where the enemy jungler is at, secure objectives like dragon and baron, ect.
Yes pink wards can be destroyed but is it because they are so easily spotted or is it your placement of your pink wards? Make sure you are placing them in bushes so they can't be seen by enemies as they pass by. Also if you place them in routes not usually traveled you will still get slight vision of them as they pass by onto another route.
Pinks are key to winning games when used right. They are worth the gold for the information you can gain from them even if it is only for a short period of time. Its not so much of how much information can they give you over time, but what can you do with what information they give at that time.