The DM, like any umpire, needs to be CONSISTANT in their rulings.
If you allow one player to have something but not another, it looks like favouritism, the players will quite justifiably get pissed off, and find themselves another DM..
Good. I wouldn't want those whiners playing with me anyway.
And no... not all DMs NEED to be consistent. Some of us have the capability to make two different rulings about the same exact thing at two different times and still maintain a fun and challenging game without pissing the players off. Partly because we play with mature adults who are flexible, and partly because we know how to make reasoned decisions and can explain our reasonings quite reasonably.
I feel sorry for any DMs out there who feel they need to maintain a lockstep catalog of every ruling they have ever made lest their players revolt in disgust.