I'm always a fan of the PTR's, because I love the chance to experiment with different specs (though this has been less of a problem of late since daily quest gold pays for a lot of respecs), but I'm especially a fan of PTR premades. Not because I like to go zerg enemy capitol cities, but because premades allow me to try different character classes before spending a lot of time to level them.
A few months ago, without the benefit of active PTR's, I posted a speculative rundown of all the classes I have yet to level and my interest in trying them out in the future. I basically talked myself into trying out the shadow priest, and I proceeded to level one to 20 on my retail server. In the mean time, the PTR's came up. A week and a half later, I was finally able to liberate a set of premades from the copy queue, and use them to test my rankings. (There were five classes on my list and I only had four premades available, but I'd previously tested both soloing specs of pre-made Shaman, so I copied the other four classes.)
Lo and behold, I didn't like the level 70 Shadow Priest much. This puzzled me, since, again, I was happily leveling one just the other day. Meanwhile, I enjoyed both stealth classes substantially more than I expected to, and I'm even having some fun messing around with Warlock builds (My Warlock alt has been indefinitely parked at level 23 because I didn't find it fun).
1. Theorycraft is not experience
Obviously, there's no real substitute for actually playing a character class. I just got Mind Flay on my priest, and I'm at a point in the game where I don't really need to worry about casting it more than once per fight. Level 70 mobs require multiple casts, which forces me to deal with not losing the channeling bar due to damage.
2. Abilities are gained one-at-a-time for a reason
One of the big issues I had with the premade shadow priest was a sudden abundance of new abilities. Half the spells I'm using for soloing at level 20 aren't available in shadowform, and there are some new ones that I've suddenly been handed all at once. There are some interesting implications for Death Knights here, as DK's will presumably have a similar number of abilities to other classes at level 80, but will have to obtain them over a mere 25 levels.
2a. Because you don't have all your abilities yet, you can't make a fully informed decision on classes at low levels
Case in point, the druid. You don't even GET cat form until level 20, and you don't get the really good abilities for Cat form until even later (notably Pounce at level 36 and Mangle with a whopping 41 points in Feral combat, obtainable no sooner than level 50).
The Warlock is another example; I'd rather play WITHOUT a pet tanking for me, and that option is available via Demonic Sacrifice... but realistically you're going to want to be able to drain tank before you try this, and that means being level 45-51 (depending on whether you feel that Siphon Life is necessary for the combo, Destruction+Demonic Sacrifice builds want even higher levels). Level 50 being notable because, by this point, you could have to the stupefying power of the Felguard (I couldn't freaking pull aggro off the thing on a premade decked out in S4 arena gear!) instead of a gimmicky petless pet class build.
Point being, this is potentially a LONG way to have to level a character before you find out what it's "really" like. One wonders if there's a good way to reconcile these two points.
3. Does the gear make the class?
In addition to the time lag before you get all your abilities, choice of gear matters to many classes. The pre-made druids copy over with Resto gear, 250 badges, and basically unlimited arena points to get the non-rating-required arena armor (3 pieces of S3, one piece of S4, a relic, and an off-hand for casters). I tested my DPS once before I hit the arena vendor and once afterwards. Needless to say, there's a big difference in feral DPS there. A player who leveled up one way (for example, a newly level 20 druid wearing caster gear) might conclude that their new shape-shift forms sucked (as I did, about four years ago while WoW was still in beta). By the same token, these premades are far far better geared than any alts of mine would be. If I actually do roll up any of the above, I won't be killing nearly so quickly.
4. Premades have a value beyond testing
Obviously, I'm not really looking for bugs here. If I find any, great, I will certainly report them, but things will be pretty bad if there's some bug that pops up in between the city of Shattrath and the Barrier Hill Ogres right outside town (which I have always used as convenient spec testing target practice). That said, my PTR experiences are causing me to reconsider some classes I wasn't planning on leveling, and clearly I'm not the only person interested (since it took a week for the queues to even allow me to REQUEST my premades, and another five days for them to appear). There's definitely demand here. I'm not suggesting that the game should have persistent premade servers (huge portions of the playerbase would never level anything ever again if given the option, while MANY players with level 70 characters voluntarily level alts the way things are now), but one wonders if there is something that can be done about this.
I could go on, but I think my hearthstones have cooled down while writing this, which means I can do another round of spec testing. ;)