[AccessD] Access & Multiple Users

I've got an Access 101 question...maybe 102...
 
I've been programming in Access for more than 10 yrs now, and early on I'd learned that, if I wanted multiple users to access my DBs (i.e. most of them), I needed to split them into Front-End programs w/forms and such, accessing Back-Ends which house the data. This is the way I've been doing things for all these years. BUT...I never really ever got any definitive word on how necessary this was...or at what point. 
 
For me, I like how I do it. It isn't much more work, and I've enjoyed a great track record, w/very few problems regarding corruption, accessibility, etc.. But, now I am faced w/an issue, and I want to back up what I am telling my users w/some solid information...actually I just want to know that I'm not blowing smoke and I know what I am talking about...
 
We are in the process of migrating over, from a Novell network to a Windows AD network. We just moved out 2nd of five servers/campuses over and this one apparently has many Access DBs on it. More troublesome is that it has many user-created DBs on it. All the DBs I created are fine...nice feather in my cap there, eh?! But, there are several issues...at least a half-dozen at the moment...w/the user-created ones. 
 
Most of these issues seem to be that they were previously being used by multiple people at the same time, and they can no longer do this. I've told the person that I am in contact w/over there that, they aren't really meant to be operated this way, so there really isn't anything for me to "fix." 
 
So, this brings us to my questions...
 
1) What IS the rule w/simultaneous access? Is it correct that this shouldn't be done...is there a size or something that it will work up to?
 
2) for my own curiosity...why did it work w/Novell, but w/it came over to Active Directory it now longer worked? I'm guessing since it is all MS now, it is integrated enough to be know better...???
 
I'm guessing this is a mix of Access versions. I'm sure most of mine should be at least at the 2003 level, but nothing much older than that.