Well, those UK-wide issues that currently aren't devolved are either more than just a few wee bits (like, y'know, defence, etc) or those Scottish MPs have no real Westminster legitimacy anyway.
Either they represent people who still have a legitimate interest in UK affairs or they don't, but either way, they have no democratic mandate to help decide what affects other people when it doesn't affect their own constituents.
It is a disgrace that devolution was enacted without resolving this issue as part of it.
Of course, it'd have been a bit tricky when we ended up with a PM that was a Scottish MP for a Scottish constituency, and that couldn't vote on a variety of devolved issues.
But I don't want to drag the thread off into a discussion of the West Lothian problem, 'cos that really isn't what it's about. I guess it's part of it, but far from the central point.