Of course, that isn't necessarily a problem. If you can make an acceptable design that works in IE6/7 (targeted by conditional comments), even if it isn't as good as the fully fledged version that you serve to 21st century browsers, that can be fine. Very few visitors to a site will check how it looks in different browsers, so the fact that it looks different in IE7 to more modern counterparts will rarely be a problem.
The problem with using off-the-shelf grid layouts is that they usually involve massive code bloat, which makes your pages slow to download and sluggish to use. And then, when anything doesn't work quite right, you'll struggle to find what needs to be changed, because it's all someone else's code that you don't understand.