I guess you are somewhat right but not on the point. If you wanted to argue that single target immobs are useful, you would have said something like: I love my single target immob, because [insert reason here why you don´t want one mob to move around while the others are free to go].
As I see it, the single target immob effect is very situational, especially for a dominator who does some of his best damage at melee range.
What you want as a dominator is a fast animating/recharging attack, regardless of the immob and that is what you get, sometimes with even more side effects (-fly being very popular while -knockback has cost more lives than it saved).
As a mind/psi dominator I like Subdue very much, but if there was one thing I could change about this power, I would remove the immob part.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
ST immobs aren't that useful in PvE, but they are very useful for PvP, especially for plant control which needs -fly for it's hold to be useful.
I actually find that it comes up so often and is so accurate that after only a short time pretty much most mobs in a spawn are immobilised, just as much as if I'd used an AoE immob, but with the risk much more spread out. Which is a Good Thing however you look at it - the enemy ain't getting into melee range of the other squishies for a start, and they are forced to use their ranged powers.
Meanwhile that lovely Domination bar is climbing, climbing ... MWAHAHAHAHAHA
With Controllers I've usually skipped the ST immob, and gone with the ST hold and mass immob (as part of "ghetto holding" in combination with the AoE "soft" control), but this experience has forced me to re-think that approach for Doms.
It depends a bit.
I just love the ST immob on my ice domi. Stacks slow and - recharge and that's golden. The forementioned building up domi is a nice side effect too as is the not bad base dam with fast activation and good recharge.
And indeed i can keep nasty lts and bosses stuck for a long time around the corner or behind a crate/box/whatever for example, get out of LOS while the rest of them is getting hammered 
But i have to say, non of my other domis have taken the ST immob, but chillblain just pops in the area of goodness because of the secondary effects.
The ST immob is very useful in PvP, it's less costly in endurance, it's much more accurate and faster to cast. That said Roots + Seeds of Confusion + and AoE/Cone Assault attack go very well together in PvE.
There is a little twist in PvP with the AoE immobs - someone without AoE defence will not defend against the AoE attack whereas they might against the ST attack.
I would've bet a horse that you were going to refer on the "1 immob works on a boss!"-thing. If a bloodthirsty Warden is after you, and you haven't got enough time to stack the hold, using the immob works wonders, as they're mag 4 -> immob a boss.
If you're going Plant/ then the immob is good for it's -fy effect which is great against those flying... things, like Wardens and Boomers.
In PvP it gets hairy because if the opponent is flying, and you NEED to -fly him with either Impale or Entangle, often the Immob isn't slotted for immobdurations (well at least mine isn't) so it might screw up suppression, making your Hold much shorter duration it already is.
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It depends a bit.
I just love the ST immob on my ice domi. Stacks slow and - recharge and that's golden.
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I was thinking of doing the same thing on my ice dom. I wasnt aware it messed with hold supression though.
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Having a plant/fire at level 38, I ultimately got rid of my immobs as I found I didn't need them once I had gotten a steady attack chain with my secondary.
Besides, excluding the -fly component, the Fly Trap and Carrion Creepers have immobilizing powers so I didn't feel the need to take it.
@Under Control
I used the ST imob to build domination early on, but they are far more expensive in terms of end than assult powers, so I respeced it out when I had enough assaults to build a decient chain.
I really should do something about this signature.
Lots of people heap scorn on single target immobs, but I think they have a good use for Doms, at least for the the Plant/Fire I recently rolled (I fell in love with the AT with a Plant/Thorn, and realised I wanted more damage so re-rolled as Plant/Fire).
What it really is useful for is building up Domination. The single target immob comes up quick, is accurate, and with 3 recharges it fires off very quickly and regularly, regularly enough so that it can be on autohit and and left firing off pretty much randomly, no matter what else you are doing it "fills in" the attack chain, pretty much at the rate of Psi Dart actually (I rolled a Mind/Psi just to check how fast PD is).
Any thoughts?