Its an interesting question... For many photographers IS on a standard zoom isn't a feature that would be worth an awful lot, as the extra stabilisation means that you can handhold a steady shot for far longer than your subject is likely to stay still (unless you're a still life or landscape photographer, in which case using a tripod isn't likely to be an issue). Of course this isn't the case when you think about a lens like the 70-200, let alone the supertele lenses, where IS is a huge advantage for stills.
On the other hand, for video, having IS in a standard zoom is still a really sought after feature - you're (mainly) stuck at a shutter speed of 1/50th, and IS is thus really really useful. As someone who does more paid video than stills work I would be prepared to pay extra for a 24-70 IS. But it would have to offer a superior performance/price ratio to the Tamron (which is sharper than the Canon 24-70 mk 1 and costs half the price of the mark ii). Given that the mark ii is now available for under £1700, I'd consider a 24-70is at £2k - but at 2.5x the price of the Tamron it would have to be a spectacularly good lens to justify the cost. I mean for the £1200 difference I could pick up (for example) the Samyang 24+35+85mm f1.4 primes as well.
