I understand why people find the convenience of DS appealing, i myself am both an avid raider and pvper, and even tho i wanna keep raiding in tbc, i will focus my attention to pvp. Despite that tho, i dont think DS is a good idea.
The problem is whats been mentioned a few times, yet people seem to ignore it/cast aside as a none issue: meta gameplay.
Ive read a few times now from people, that they are confident DS wont force people into running 2 pve specs, for optimal raiding use, bc ppl dont care that much. Im afraid this is very far from the truth… It does not take more than to look at what happened in classic, to realise how wrong this is.
In classic a lot of us said, we were not gonna get world buffs and raidlog, we would not speed run, min max and what not. Yet after a short while, most decent guilds started building their roster, specs and what not in accordance to what the top guilds did - because they wanted to be wannabe tryhard. They wanted fast clear times, short raid days, efficiency etc. The meta mentality from top guilds trickled down to most end game clearing raiding guilds, so i think it is very naive to think that is not gonna happen in tbc.
Top guilds will definitely utilise DS for speed clears. Maybe everyone will be required to run 2 differen specs (ex. tank/healer), but more likely they will just run 2 different variants of the same spec, so they can swap on the go for trash/bosses, or boss/boss to whatever 2 specs gives the best results. Sure they can do that now by heartstoning back, respec and get summoned - but that would be very time costly, and probably not worth it in a world first race, or a speed clear attempt. In addition, you wont be paying 50g every time you respec if its through DS, making it even more likely to be mimicked by less hardcore guilds.
So i highly doubt any top guilds will be running 1 pvp and 1 pve spec. They will just respec to pvp after they raided in one of their specs. And as time progresses, more and more guilds will adopt this mentality as well, forcing you to pay for respecs to pvp anyway. It will be come meta, wether you care about it or not, and that is the problem for a lot of players. It might not matter to more casual guilds, but considering how many guilds label themselves as semi-hardcore+, it will impact a lot of guilds, just like world buffs did.
What happened in original tbc is not really a good reference. Back then we didnt know what was gonna be released, we didnt know the optimal strategies and boss values before the raid even released. This time we know everything on day 1. Just look at classic - vanilla sure as hell did not get min maxed to this level we have today, and people was not as vocal about it.
We dont go into this expansion blind like back then, we know what is gonna happen, and due to our modern days meta mentality, we will uptimize what can be uptimized. The only thing serious raiders care about these days are clear times and parses, even tho its a bit sad because the content is so easy.
So tldr: We dont go into tbc blind, we know whats gonna happen, and classic has proven that top guilds will uptimze what can be uptimized, creating new metas that the majority of serious raiding guilds will adopt - as such DS will create a terrible meta problem, and not really solve the problem it tries to fix.
Anyway thats my opinion, you are ofc welcome to disagree. But i hope what i said can start some reflections, and make people realise that meta gameplay is a bigger deal than what people want to believe.