The concept of BiS gear (or Best in Slot gear) is a common way to 'rate' drops and or gear. In the 'old days', during vanilla WoW I had the feeling that the BiS gear was almost always the Tier sets and raid boss drops (seen from a leveling point of view, I had just reached 56 when TBC hit). Now, next to the tier sets and raid boss drops, there are also the crafted items, the 'regular' emblem rewards and even the rare trash-mob drops. All in all its not that strange that a concept such as BiS was introduced.
But how important is the BiS gear statistic and how can one actually claim a single item to be the BiS item?
A gear set on any character is built around his class and his talent spec. The total 'package' of gear is balanced around the needs and requirements of the class and player. When you change one piece of gear, it is not uncommon that you will also have to change several other pieces of gear just to restore the balance and benefit the most from your new item. So how can you take one piece of gear out of the lot, and state that it is BiS? Did someone actually put together the ultimate total package of gear for a class/spec combination and according to that is calling each of those items the BiS items for optimal performance? Or is the BiS determined by looking at the individual items and rating them for its stats? (if anybody has more info on this, feel free to enlighten us poor sods) But are those ways of determining BiS gear of any use?
A general player will reach 80, begin to gear up and over time collect more and more items. In general its a dynamic process, where a player will look at the options, and decide on which parts to upgrade, and what parts need upgrading next. This means that when you get your hands on one of these 'mythical' BiS items you should be very happy because its the best gear for you...but is it?
When following the first reasoning (BiS determined on complete gear set) then your lacking a whole load of other items with which this item would be BiS (I haven't yet met a single player yet that just collects gear in the bank until he has a complete set and then starts wearing it). If your using the second method (BiS on individual stats) then how on earth are you going to manage your balance? I mean you already have a gear set, you replace one piece which is supposed to be BiS and all of a sudden your balance is way off (not even to mention that in one fight some stats have a higher importance).
Especially the second way is quite dangerous, for example when using Gear wishlist. For the gear wishlist you can either put together your own weighted stats, or use a general GearScore method. When using the second for Locks for example, items with Hit will always rank higher. But what if I'm already full on hit?
Now, this entire rant might sound I'm completely against the BiS concept, but I'm not. The concept is good, it gives an overall impression of the best items to go for out there. What I am against is the abuse of the term (as has happened to so many other terms). Players use the term BiS at their own convenience and do not look at the boundary conditions which are important when selecting gear.
In all kinds of chats (be it general, trade or guildchat) you hear people using the term. But how many actually know what they are saying? How many know how the BiS item they grabbed from some random website was determined?
When looking at it from a distance, it all comes down to the (in)famous GearScore again and the everlasting discussion regarding its use and abuse...