Sure.
The only downside is if there is stuff that will need to run when the engine is off. Lets say for example you needed to run those fans for a 10 minutes after the engine was shut down. In that case the battery will have to send power all the way to the distribution block and through all of the connections in between. If its only a couple of amps, it doesn't seem like a big deal. But when the engine is started, the battery will be low and charging will be high flow for at a few minutes. So that's when we have to be careful.

For example a few years ago I accidently left the parking lights on all day in my '67. After jump starting, the battery was taking all the power it could. At idle, the alternator can only supply like 10 amps. But as soon as I started driving, it was able to supply 20 - 30 amps. So what I had to do was turn the lights on, and shift it into neutral and coast wherever possible. This kept the battery from sucking so much power.

I only drove it about 20-30 minutes to get back to base camp. The battery was hot. Batteries don't actually charge well when hot, but they don't know that so they suck all the current they can when low. Next morning it was cool, but not fully recharged. We had no charger in camp. So, did the same thing on the first leg out of camp - about 30 minutes. By then it was starting to get close to fully charged. Let it charge at idle for a bit more, and then it was pretty much back to normal for the rest of the trip home.

So anyway, this is a situation that can occur anytime the battery gets drained.

But depending on the situation, sometimes it does make sense to have a direct wire from the battery to the alternator. That's when a voltmeter makes sense because its a different wiring strategy.

Nacho's situation was such that simply raising the idle speed a little and using a better alternator would keep the alternator as the highest voltage source. But all he needed to do was keep the battery from discharging when running the A/C at idle (sitting in traffic).