Cracks in the manifold are not good, they reduce the backpressure and cause loss of performance. Not sure if it would cause a rough idle though, I guess it may if they were bad enough...
Your options for the manifold are:
Repair (weld) the old one - Cheapest option but not long lasting.
OE Manicat at around £400 - will be a direct and no aggro fit, is costly and offers no performance upgrade.
4-2-1 manifold, universal cat for circa £350 - not a direct fit, will require modification to your existing system to fit. Will give something like 15Hp extra (more with an aftermarket exhaust) and looks class. No further cracking issues...
So there you go, you pays yer money etc etc