It's obvious to most people only after they have done it. First, you have to understand that any machine that has an outside DNS server's IP setup as a DNS box can contaminate its name resolution. For example, let's say you had yada.yada.local as a real name on the outside world, and as your LAN domain (not gonna happen, but this is just for theory). Now, on a given client you have your internal DNS box as DNS server one, and an outside DNS server box as DNS server two. Now, when the client needs to authenticate, the first thing it's gonna do is try to resolve the current domain that it's in, and find the first available DC to authenticate it. It will try the first DNS server, and hopefully get in contact with it. Now, in a low demand environment where the server is ALWAYS available, the problem would never manifest, but you will see the "rub" soon enough. Now, let's say it's booting up, and the first server isn't available. Well, since it's a fresh boot there will not be any sort of DNS resolver cache to fall back on, so it will hit the DNS boxes. Next, the first is a no-show, so it goes for number two. Number says that yada.yada.local is your *outside* (real) IP, and it will hand that info back to your client. Now, the client will keep hitting the outside IP thinking it has the proper information for your domain, and will keep hitting your outside IP (probably on the other end of a router or NAT system, right?) until it gives up, then moves down the resolution chain. Unfortunately, this is where you see a lot of AD errors and timeouts; simple name resolution.
So, what do I do about it you ask? Simple, you setup internal DNS box(es) that handle all name resolution for your clients, and only those clients. The forwarders are there so the DNS server that you setup can get info on other domains besides the one setup locally on it (it doesn't mystically pickup every single domain ever listed unless you try to sync it with a root server, and you *really* don't want that) and can then help clients get to where they are going. Normally, I suggest putting your ISP's DNS servers on there, as they are faster to get to (hop-wise). You can use root hints to bypass the ISP's servers and go straight to the source, but I never bother.
This is what I do. I setup the DNS box for my domain, and enter static assignments for my servers and other fixed IPs (printers and such). I also enter WWW, FTP, SMTP, and MX records as needed. Then, I enter all the official DNS boxes on my network into the DNS Servers tab, and then instruct them to only replicate with servers listed on that tab (that way your boxes don't arbitrarily receive traffic from the outside and try to replicate junk data from your non-routable IPs to the outside world). I also enable WINS and WINS-R lookups and point them to my WINS server (yeah, it's supposed to be dead but sometimes you still need it).
If you need more info, let me know. But, this should be enough to get you going. You can also do a search here using my name and "DNS" for other explanations on this.