You are asking for opinions, so here's mine:
To start off, I'll say I usually bulk age well over 12 months. However, this may be changing for me.
If you want to make sure there is no sediment in your bottles, bulk age for a few extra months before bottling. This is a great reason. I would recommend doing this.
If you think you may make some adjustments during bulk aging, like added tannins, acids, Biolees, ..etc., bulk age until you are satisfied with the wine.
Long term bulk aging lets you taste your wine as it ages, so you can better determine when the wine starts coming around. Otherwise, you have to have bottled several splits or you have to be willing to pop the cork on a full bottle, which may not be ready to drink.
Other than that, there really is not much reason to bulk age. There is nothing special that happens during bulk aging that won't happen in the bottle... that is, nothing good.
I read this in another place but there is something to be considered about leaving a carboy of wine under an air lock for over about 6 months. Some do believe that much of the aroma escapes from a bulk aging wine through that air lock. I am NOW one such person.
I recently taste tested 3 of my wines, still bulk aging and each has been for about 12 to 15 months. Each has had an air lock installed the whole time. I noticed that even with a lot of swirling in the glass, there seemed to be almost no nose left at all. I do believe over a long period of time the aroma is escaping through the air lock (and around the bung).
So, if you are going to bulk age over 6 months, after 6 months consider replacing the bung/air lock combo with a solid bung.
Something else to consider, even with a solid bung - Tim Vanderbilt (Check my spelling) of WineMaking Magazine says that the bung in a carboy, even if inserted properly, leaks a lot of air over several months. I don't know how many air exchanges that really represents, but over time, apparently enough to bleed away a lot of the aroma of the wine.
As temperature and barometric pressure change, around the bung, air sneaks in and out of the carboy to compensate. Undoubtedly, some of the wine's aroma is sneaking out with it. (This is one of the few advantages of a Better Bottle over glass, the bottle can expand and contract without it becoming necessary for air to come in and out through the neck of the bottle in order to compensate for pressure changes.)
I have been writing about bulk aging in a carboy and have not considered a wine barrel. There is a whole other set of dynamics associated with bulk aging in a barrel. The barrel is very leaky, but in a good way, not to mention the oak flavor it can impart to the wine. It lets out CO2 and water; lets in micro amounts of oxygen. These are things most of us want to happen.
To wrap up my opinion, don't bulk age just to be bulk aging. If you are not going to make adjustments, leave the wine in a carboy an extra month or two to drop out any extra sediment,. and bottle 4 to 6 splits (375 ml bottles) for sampling. Other than that, if you don't have any other specific, good reason to bulk age, don't.