Downsides to the idea
1. Lots of hours on the main engine
which will wear out sooner and cost a lot more to replace than a generator
2. Also, not good to run your diesel
at low rpm
with a light load for long periods.
3. Most boats, running the main engine is pretty noisy as well.
4. The alternator
would not be able to supply the power needed so would slowly deplete your batteries.
5. To get an alternator big enough to supply the load would cost a lot of money
, especially if you get one rated continuous
duty to put out a couple hundred amps. And it would take a serious mounting bracket AND unless you did something to counteract (serpentine belt, opposing idler pulley, etc) you will be putting a large side load on the crankshaft that could cause early death to the front seals
and maybe even bearings.
As a very occasional way to run an AC at anchor
it could be an option but wouldn't do it on a regular basis.