If you're accurate enough not to pee on the counter then the only thing holding back someone so inclined is the social taboo. It's not like the kitchen sink where stuff regularly goes in there. Does pee harm pipes? Will it make the sink bowl radiate a permanent smell? I just hate wasting water by flushing only pee but also don't like letting pee sit for an excessively long time.
splashing my face with water, or drinking from the faucet to rinse after brushing. i would stick my head into the toilet to drink from a bidet, so making your sink a toilet would be gross as hell.