In short, your last sentence pretty much says it all. You shouldn't really be using Dugtrio outside the roles of revenge killer and Chansey / Registeel weakener. Outside of those two niches, Dugtrio is frankly terrible. However, it is because of its ability to fulfill those two niches extremely well that its tiering status is being questioned. A Pokemon does not need to fit perfectly into one of the "Uber" characteristics to be voted BL. They are merely guidelines.
I really must wonder if this is enough though, unless we want to consider moving it up all the way to Uber. After all, couldn't some of this same logic apply to its use in OU? It's great at revenge killing stuff like Blissey, Heatran (assuming no Shuca Berry/it already being take care of, since we aren't assuming Shed Shell/Shuca Berry for Registeel or Chansey here), Electivire, Jirachi, and most Infernape (looking at the stats, most don't run Focus Sash or a priority move). Thus, under that same logic, if that's what's being proposed to make it BL, then would it not also be Uber for similar reasons? This makes me wonder.
In any case though, personally, I don't feel this is enough, as it's only particularly effective against particular Pokemon in UU (namely, Registeel and Chansey). There really isn't a wide spectrum of commonly used Pokemon it can blast through to support the team, unlike other Pokemon that were banned under the Support Clause, like Latias (well, potentially at least), which can open up holes on teams by destroying any number of Pokemon--it's mostly just those handful of Pokemon that Dugtrio really shines against, and I'm just not sure that's enough, especially since under that line of logic, as I mentioned, could have us going all the way to voting it Uber were it actually followed through all the way, which just doesn't sit well with me.
Plus, the fact is that there
are things that Chansey and Registeel can do to defend themselves against Dugtrio, while not making themselves useless in other situations--namely, using Shed Shell (just as Steels like Skarmory sometimes use it to defend itself against Magenzone in OU, with no cries of Magnezone being broken due to the fact that it's pretty much the only reason Skarmory runs it at all, and, back to Dugtrio, being just another reason for Heatran to use Shuca Berry or a Scarf, where there again haven't been any cries for it being Uber). With that, Dugtrio loses its ability to trap them and thus becomes no different from any other revenge killer--a good one, but hardly a broken one, IMO. Shed Shell may not be their best option, true, but they can run it without becoming terrible Pokemon. And since similar situations aren't causing a ruckus in OU (see: the aforementioned Steel-trapping Magenzone "forcing" some of the OU steels to run Shed Shell, prominently Skarmory, a comparable Pokemon to Registeel, to stop it from revenge killing them without it being an issue (or, to put it slightly differently, there are no cries for banning Magenzone over the holes in teams it creates by beating potential defensive cores like Skarmory, and also stuff like Foretress)), I don't see why we would suddenly turn around and actually ban Dugtrio under this logic.
Dugtrio definitely is a good revenge killer, and it's great at what it does, yes. However, I just can't see the justification for banning it right now. There may in fact be good reason for banning it which may yet be discovered/posted, but looking over the discussion thus far, I'm not seeing a good enough reason at this point and time.
