On modern cars? No way, I would do the same thing. There are so many electronic things under a hood it's not even funny. When stuff shorts out, it's typically not right away, and had you not caught them, they would've told you it's not their fault/ problem. It's something you did. Good luck troubleshooting something with several computer control modules.

Even on old cars it's not a good thing to get the engine area wet. My '79 Pontiac only has a couple relays, no computers under the hood. It can still short stuff out. A battery got blown up under the hood and corroded the crap out of the horn leads. It took a while to figure out why it didn't work. There aren't many wires, but there's a reason the hood is there. 

I’m with @mmj. Nothing to worry about. Don’t sweat the small stuff

Even though there is a bonnet, everything under the hood was designed as if it were an exterior part of the car. Unlike the vehicle interior, all electrical connectors under the hood are weather sealed. Fuse boxes have interlocking lids. Electronic control units are in tightly sealed housings, etc. The under hood area has to be able to handle getting completely soaked. You will notice that both the front, bottom and sometimes sides of the engine bay are exposed to the road. Driving through water can splash water everywhere. And sometimes cars need to drive through deep water.

Rain should not hurt anything. In my opinion, yes you are being fussy.

https://www.youtube.com/watch?v=ikdle3u51l4

Posted by: @justin-shepherd

On modern cars? No way, I would do the same thing.

Posted by: @mmj

Rain should not hurt anything.

Your car, your call, that simple.

I personally would NOT appreciate a mechanic leaving the hood raised allowing the engine to be rained on if they did so knowing it was going to rain.