To be very honest its mindset like this that has created such a lack of good spin bowlers in Australia, people like John Davison (FFS), are teaching spin bowling to aspiring spin bowlers.
Dean Jones recently said that CA should seriously think about hiring people like Saqlain and ask him to coach young Aussie spin bowlers, and that's definitely something that should be done, a lot of spin bowlers in Australia hardly know much about their trade, because most of them aren't being coached by the right people.
I think all these "spin mentors" really need to wake up and smell the coffee, they need to realise that's its year 2009, and every finger-spinner around the world is either bowling a doosra or trying to develop one, because since the pitches are flat like ****, they know they won't last long without having any variations.
Regardless, my point still stands, it is possible for finger spinners to make a go of things without the delivery.
And I do not agree with your reasoning of "If SLAs can, surely offies too can without the doosra". First of all how many SLAs are there in the history of test cricket who can be termed as world class? (say have taken above 250 test wickets at a good average?) That in itself should show the longetivity issues of LSAs.
Left-arm fingerspin has always been constrained by the same limitations as right-arm fingerspin. Given a non-turning surface, neither will have any effect against quality batting.
However, given a turning surface and (and this would be regarded as the standard) a surfeit of RHBs, a left-arm fingerspinner would generally be a decent bit more effective than a right-arm fingerspinner. This is the reason why, among the groups of fingerspinners who've had regular access to turning decks (ie, generally England before 1970, and India\Sri Lanka\sometimes-Pakistan), left-armers have generally abounded more than right-armers.
Almost all the few good ones from elsewhere (Tayfield from South Africa, Gibbs from West Indies, Johnson from Australia, for example) have been right-armers. Quality left-arm fingerspinners from any of the non-subcontinental countries (and England up to 1970) can probably be counted without too many counters: Vettori for example, or (briefly) Valentine.
As for the Doosra, any fingerspinner can bowl it. Same way no left-armer has yet bowled wristspin to a remotely notable standard, no left-arm fingerspinner has yet produced a Doosra (so far only Saqlain and Harbhajan of those with any decent skill have produced it of the right-armers, let's remember). But as long as they keep trying, it'll happen eventually.
Starting to struggle, itbt...
lol left armers can't bowl the doorsa. I guess you have never seen Hearth bowl then?
And as for Michael Bevan who you edited in, the inclusion of a batsman whose bowling was about as inaccurate as anyone I've ever seen says it all.