Why grind tickets, you ask? The two "big ticket" items, costing 1200 each, The Orb and Amulet of the Darkmoon, have admittedly lost a bit of their luster these days. They were incredibly nice back when Darkmoon Faire was new, mind you. As a feral druid, I wore my amulet with pride for months, right up until I found a new necklace in Hellfire Peninsula in the first few weeks of the Burning Crusade! These days, that means you'll sadly probably find a better necklace somewhere in Outland long before you save up 1200 tickets. While we can hope for new holiday upgrades, for now the ticket system could use a bit of new life. That said, there's still reasons to gather tickets.
First, there's still some loot to be had. For 5 tickets, 12 tickets, or 40 tickets, you can grab yourself a Darkmoon Prize, which will contain trade skill supplies, a recipe, a green item, or even a blue item. It's, at the least, a nice way to satisfy the gambler in you as you level up. If you're an engineer, you'll want to grab the schematic for the Steam Tonk Controller so you can hold Steam Tonk matches while waiting for those last few stragglers to show up for a raid or for your arena queue to pop, and 40 tickets aren't too hard to get. Finally, bag space is often a struggle for the leveling (or non-leveling) character, so an easy to get 14-slot bag such as the Darkmoon Storage Box could be -- excuse the pun -- just the ticket! While the box is unique, so you can only get one, it's still trade-able, so if you ever find yourself no longer in need of it, you can pass it on to one of your other characters, or a deserving newbie you know. Of course, if all that fails, you should never underestimate the comedy or role play potential of waving around a haunch of mutton or two, if you can equip maces. In fact, I'd say that allowing your warrior or rogue to swing around two flaming pieces of meat might be the best possible use of 2 Essences of Fire and 8 Small Radiant Shards ever conceived.
The second reason is that it can provide great synergy with leveling up of trade skills. It seems like every trade skill has those points where you need to make the same item over and over to level up, but you just can't seem to find anyone to buy your wares other than your friendly neighborhood general goods vendor. That's where, if you're a leatherworker, blacksmith, or engineer, the Darkmoon Faire comes in to play. I recall that when I was leveling up my engineer, I made more than my fair share of Thorium Widgets on the way to 315 skill, and being able to send those widgets over to my druid to help him get his amulet was a great way to dispose of them when they didn't sell very well on the auction house. Likewise, my leatherworker spent a lot of time skilling up on Rugged Armor Kits, and Chronos took those from me for even more tickets. Check out WoWWiki's Darkmoon Faire ticket guide for a good, concise list of the various turn-in quests, and to figure out which trade skill items you can make.
The third reason is that you can also use the gathering of various animal parts for Yebb Neblegear to help you decide where to level up. For example, the famous Jame's alliance leveling guide suggests heading to Thousand Needles when hit level 30 and gathering Vibrant Plumes off the Harpies there, 25 of them to get enough tickets for the aforementioned storage box. You might also look for them on the various Moonkin of the Hinterlands and Winterspring, or on other groups of Harpies like the ones up in northern Feralas. I'm personally a fan of leveling up in Desolace and the Badlands, so I often get Soft Bushy Tails for him. When I'm leveling Horde-side, I always seem to end up helping the Warsong clan in Ashenvale and the Royal Apothecary Society in Hillsbrad, so that always give me a good excuse to collect Torn Bear Pelts as well.
The last reason is a bit more simplistic: You just like seeing that green reputation bar inch on up. Sure, it takes a few (thousand) widgets and armor kits, but hey, if it means rock bottom prices on Cheap Beer and Darkmoon Special Reserve, it's got to be worth it, right?
By the by, for all you less than level 58 types out there who are a bit bummed that the Faire's in Shattrath, don't give up quite yet. You can't head directly through the Dark Portal for sure, but if you can hire a Mage, they can still portal you directly to the city, and from there, it's a very quick trip outside, down through the lower city and out the southeastern tunnel. Plus, you can go to one of the city's Inns on the Scryer or Aldor tiers and set your home to the city, and have an instant portal to any other capital via A'dal's room.