The value of one's upvote is determined by three things: The HIVE in one's account, their voting power and the size of the rewards pool.

The rewards pool comes from a fixed interest on HIVE. One's voting power is not exactly wasted when one's voting power is at 100%. What happens is that voting power of others increases a smidge.

The value of our vote would drop dramatically if everyone kept their voting power under 100%.

I don't know if you remember the hardfork in March. This decreased the number of votes which temporarily increased the rewards pool and the power of everyone's vote increased dramatically.

Leaving one's voting power at 100% is a little bit like transferring HIVE to @null. The action benefits the community at large.

Of course one loses curation rewards when one's voting power is at 100%. The lost curation rewards get realized by others.