Jezbot, that is all true what you say but the "firepower" of both units is not their selling point. It's just a sideshow. Of course I'm going to shoot both if the opportunity arises, it's just nothing to write home about regardless of whether it is a distinct role or just an aspect. I think we all agree that every unit has to be put into some perspective, so I can only repeat what I've said above: If you see a relevant amount of high-value, little T/armour targets like 10 Swordmasters, then Pistoliers are an excellent choice for their pistols alone. Since this does not happen very often in my neck of the woods, they could as well be armed with kitchen forks.
KimikoPT, tarpitting is indeed their most noticeable forte, and not a job to be underestimated. They are not only the most effective tarpit, they are often also the most cost-efficient one. Sure, one can use stubborn GS, detachments, busses, Reiksguard and whatnot these days but each and everyone of them is either more expensive, easier to wound, less resilient and more likely to break, or a combination of these. You can run a STank right into a horde of GW Graveguard and expect it to stay there the rest of the game. You cannot do that with 5 Reiksguard or 25 Greatswords or a stubborn detachment.
This means that the STank is not only better at conserving points. Holding out round after round has a lot of tactical implications such as blocking approaches to more fragile targets, locking down whole flanks, locking down more points than you have used, fixing a nasty character into place and so on. This ability alone can and has won games.
Thirdly, the STank is not bad at dishing out damage. Perhaps they'll address the issue of grinding in the movement phase, perhaps they don't, but that can amount to a lot of S6 hits. Pretty good against Brets or Chaos Knights, Rat Ogres or other relatively small units which would nevertheless make a mess of a block of Halberdiers. We also rarely expect to break enemies with a single other unit. Everyone talks about how Empire is combined arms. Why then should it be a concern that the STank might not break enemy units? What if it "only" kills a third of a horde of Bloodletters or White Lions? That can be enough for the infantry to take over.
Fourthly, there are the guns. Of course you can field 20 Greatswords or 40 Halberdiers instead and hope to emulate the tarpit function (which for 250 points is impossible, you usually need to figure in parts of the cost of a General/BSB and their availability as well). Even if they could do that, they don't get a cannon on top. And even if you'd buy a cannon, you still haven't got a Steam Gun into the package! With a potential S4, the STank goes a loooong way to reduce ginourmous hordes to a more manageable size with one shot.
Lastly, albeit minor points, there are the Engineer's repeater pistol and the option to take a moving Hochland for the first turns or when you don't wish to generate Steam Points, so it is a really really good deal overall.