I see absolutely no reason not to allow material from dragon mag unless its obviously broken.
While on the subject of allowing non-core material, ive noticed several camps of thought :
-CORE ONLY!!! This is nearly always divided into 2 sub-camps :
1.Anything in the holy grail(core books) is obviously balanced, anything that isnt is obviously imbalanced. Or they just dont like the idea of using non-core. Basically if wizards re-printed the core books and included material from supplements in them, suddenly its okay to use them, simply because it is now printed in the core books.
This school of thought makes absolutely no sense to me.
2.Keeping it simple. The DM wants to keep things simple, and restricts stuff to core only to achieve this. Personally i disagree with this approach because you are killing lots of variety by doing this.
-WOTC official only. Any official WOTC book is fine, but third party books are not. Again this makes no sense to me, if wizards suddenly buys up all those companies and releases their supplements as "official WOTC", it is now perfectly okay to use them, ignoring the fact content didnt change at all.
-Campaign specific material is out, because it is obviously imba in another campaign world or "doesnt fit", the latter of which is easily handwaved in all but the most unusual of situations.
-(Very common) Core + list of commonly used books, such as the complete series. The most common reason is "these are all i have", but sometimes the DM is trying to restrict it to a small list of books for various reasons, ranging from "keeping it simple" to "the rest are too imba".
-(Most common) The "everything goes, but use common sense" approach. Assume everything is allowed, but if something is broken, its not going to be allowed. This also includes the "pretty much everything is allowed, just let me know what non-core stuff you are using so i can look it over" approach.