Some thoughts on Moat.
1. Moat is an engine card
This is important to remember. Moat will hardly ever be any good outside of engines. There will virtually always be something better than Moat-BM. Other deck types have even less use for it. Don't just buy Moat in a non-engine deck because you want to block attacks; in almost all cases, it will be better to just get the attack yourself instead.
Thus, Moat is only ever worth getting with a splitter around. Other cards that help Moat are trashing and an +buy. There are board where the engine is only possible because of Moat; in that case, you of course need to get it (unless you can do something better). The more intestesting question is: Do you want to add Moat to an engine that could also be built without it?
2. When to defend from attacks
The key utility of Moat is of course its reaction. But getting attacked does not automatically mean you need Moat.
Junking - here, you should probably have a way to trash junk if you want to build an engine. And usually, the better way to defend yourself is to get more trashing. Or, you know, junk back. Moat versus junking mostly won't work out, unless you can set yourself up a thin deck really quickly but the opportunity cost of trashing more is high (e.g. with Donate).
Discarding - It's still probably not great. Moat is one card in your hand that could be something better, so you've discard-attacked yourself a bit. A better way to defend against e.g. Militia is to build more so that you can get going from a 3-card hand. But some attacks are really nasty and it's worth blocking them; Ghost Ship and Legionary deserve a mention here.
Trashing - Probably the nastiest attack type for engines. If you expect to get hit by trashing regularly, Moat can really help you out. Additional benefit is that as a $2, Moat itself is mostly immune to these attacks, and helps you neutralize them even when it's not in your hand. This is a case where you really might consider using Moat instead of some stronger draw.
In general, the more (stacking) attacks your opponent plays this turn, the better you Moat gets. Trashing is particularly important to prevent. Moat of course also benefits from cards that can make sure it's in your hand every turn (Scheme, Harbinger etc.)
3. Moat is cheap draw
Sometimes it's really all you need. This is true whenever there's an overabundance of +action. Like all terminal draw, Moat can be a great Lost Arts target. Champion + Moat seems redundant, but it's actually quite decent. With enough Villages, two Moats for $4 and two buys might even be better than a Smithy.
This sounds nice, but the reality is that Moat is one of the worse ways to add drawing power to your engine. If there is another way, and it's not something outlandish like Beggar/Spice Merchant, you're usually better off going for that.