What any parent wants I feel is Peace of Mind when their child is sleeping. Baby Monitors are common place these days - yet back when I was little, I don't think baby monitors were around (anyone know if they existed in the 70's?). Even with a baby monitor, in a party situation, would you be able to hear it... would it work over the distance (party room may be some way from your hotel room), and would you be able to respond to it quickly.
You always have added Peace of Mind when in your own home... but when you are somewhere new, you will be on your guard. Therefore to enjoy the wedding party, you will want to have the added Peace of Mind having a babysitter gives you.
As you may be using a babysitter who is unknown to you, and as you are closeby, then you may wish to pop back to your hotel room to check everything is going well. Having a babysitter will give you the added peace of mind, only if you fully trust that babysitter, so spend time finding someone suitable, ideally get them to meet your child before bedtime, so that if your child wakes, they don't find a complete stranger attending to their needs.
I can see that your parents may think it is fine to leave your baby in a locked hotel room, it is a different situation to leaving children in an unlocked room/apartment. But there are risks and if you can minimise those risks, then do so. While it may cost quite a bit to have a babysitter at a hotel, the price is worth paying for the Peace of Mind.