I don't think exclusive items in and of themselves are inherently good. By seeing an exclusive item, I think to myself: "Hot dang, I want that because it's cool." From here, there's two potential paths. There's the "Darn, I guess there's pretty much nothing I can do to get that item..." path, where the item is unavailable for some reason or another (like needing to wait 11 months for another chance to get them). Then there's the "I'm going to go out and get good so I can earn that myself!" path, where the item is available but tough to get for some reason or another (like being really expensive or requiring the player to complete certain difficult actions). One path makes the player feel powerless and locked out of a good experience while the other one give the player a potentially daunting yet likely enjoyable challenge to overcome. One path makes the player sit and wait for the circumstances to fall in their favor, the other makes the player go out and learn to do cool things even if the circumstances are against them.
This is why I prefer achievement-based exclusive items to time-based ones. Time-based items don't incite the same passion that achievement-based ones do. Time-based items don't open up the potential for amazing stories quite like achievement-based ones do. Remember how people used to complain about the number of people in the Casino? At least they were actively trying to earn some money, even if it's not the most skill based way to do so. Now we have people idling along the beach, waiting in thirty minute intervals to get their next tickets to the Halloween slot machine. It's not that this isn't fun (the atmosphere and character visits certainly keep the game fun, and the social aspect means that waiting usually isn't a boring activity), but I don't see myself getting any heroic stories from earning the Halloween items quite like the stories I've gained from earning GMT milestones.
I could equip my green flame and recall the time I narrowly escaped the infected as the last alive wielding nothing but a shot of adrenaline and a double barreled shotgun, running around with one eye on the radar and another on the surroundings. Infected right on my tail and more coming from every angle... except for the floor below. With a sudden turn and a dramatic leap, I evade the infected long enough to spin around and finish one with each blast as the timer reaches 0. Another person might have been able to equip their Chimera pet to recall their dramatic last stand as the last pigmask against the dreaded beast. A stand of jukes, roars, tailswipes, and missed bites, only to climax at a heroic crouch leap over the chimera into a daring button press to end its reign of terror.
These are stories that happen naturally. While seeking exclusive items might not explicitly cause them nor is it a requirement for them to occur, but when you're looking to do something seemingly impossible you'll accidentally do a few cool things along the way. The items you earn serve as testaments to these feats, even if the feats themselves don't unlock the items. I can be proud of the Virus flame I earned, but I can't really say the same for the daggers I happened upon.
I guess, in the end, it's not that I hate time-based exclusive items. I just don't think they have the same kind of positive motivation that achievement-based ones provide, and being barred from getting any item is always a bummer.