I guess if the post is really deserving it is not an abused of the system anyhow in my personal opinion if I was going to merit somebody if I see he gain more than 100 merits already to his post I won't add anymore I will give it to other deserving persons who need it.
Agree, and also we can't judge the sender on how many merit he/she will send (unless if it's bought, e.g. buy & sell merits, exchange merit), that's their own sMerits so they can do whatever they want.
That's why there are some insidence about merit, like what the OP said, sending too much merit for a low quality post only, but ended up nothing.