You are telling people that it's not bad if they use this plugin. If you want to help them to get their MySQL database emptied, sure, you are helping them a metric fuckton there, but otherwise...

Doing sanitation twice, once on the client and once on the server is ridiculous. You don't have to defend against the pentester and the script kiddy separately (besides, assuming that only these two exist is just naive). Do the sanitation once and don't write code that you don't need. If you connect directly to the MySQL database... Well, don't do input sanitation at all, because its worthless anyways. Anyone can connect to your MySQL database and run queries against it anyhow, and people will do that, promised.

Not defining an API on a webserver that your client communicates with is just asking for trouble. Wether you implement that API in C, PHP, node.js, RoR... No one cares. It doesn't matter if its SOAP, REST, XMLRPC, what have you. Important is: Don't fucking trust third party, sanitize all input ON your server, not on the client.

Edit:
I feel like a fucking broken record here. I'm telling this since years in this forum, every time a thread like this pops up and everyone is like "hooray, MySQL in Gamestudio". And no one fucking realizes that this is just like going outside and handing everyone a key to your front door. Sometimes I think you people are just fucking around with me and not actually that retarded, but it appears to be that in fact you are. I realize that security isn't a state but a process and that it might not be obvious why this is a bad idea, but come fucking on, connecting from an untrusted client to a MySQL database is stupid. News at 11.

Seriously, this complete utter lack of responsibility in regards to security is baffling, and its something that is running around in the indie games scene like a kid with ADHD after you fed it a kilo of sugar and cocaine. It's not funny anymore, it's depressing and it makes you all look bad. Fucked if I know why this is so prevalent with game designers, but I have the theory that you are all just a bunch of hipster kids that sit around starbucks all day and sometimes circlejerk each other when no one looks.