When Heikki came to McLaren, it was on the back of an impressive rookie season at Renault where he outshone his experienced teammate (the same teammate, btw, who had outclassed Massa in 2004). I'll agree that last season Heikki was all the use of a sack of potatoes, but I don't agree he was picked to be a ragdoll in the second car.
He was still under immense pressure at Renault due to the amount of unforced errors he made. He was under the threat of being ditched in 07. His team mate was Fisi who Williams once spoke of as a driver they would have signed ages ago if they thought he was any good

Heiki has clearly been signed as a no 2.