mk4 vauxhall astra cracked exhaust manifold

[mbtech01]

The father in law has just got himself a clean/tidy Mk4 Astra 1.6 8V. Im in the process of doing the head gasket as it was loosing coolant & all checks led to a faulty head gasket. When removing the exhaust manifold heat shield 2 hairline cracks were visible in the manifold - one running from the o2 sensor across no.4 exhaust branch & down onto the catalyst which is part of the manifold & the second crack was seen running along no.1 brach of the manifold. Neither appear to be blowing at the present time but i'd just like to know is this a common fault. The car has done 100,000 miles but has been well looked after with a very full service history. Im just amazed at the depth of the cracks in the manifold/catalyst & can't believe its not blowing. They both look as though they have been there for some time as the hairline cracks look quite rusty & aged. I'm tempted just to leave it for the present time & replace it when it finally starts blowing as it isn't that hard to change. Anybody come across this problem or know if it was a common problem ?