I have seen a number of websites lately still using query string URLs (http://www.mysite.com/index.php?id=1) for general content pages. I do not object to using these for special cases like a complex search, product filtering and other cases which I can't think of at the time of writing this.
When I see this showing up on websites built within the last 4 years that has 100's of pages it makes me wonder if the Architect/Developer planned this properly.
Why you shouldn't do this
It's against Geneva convention of website standards ruling body. Not really I don't even think there is such a thing. The real reasons are below.
This will cause you great trouble if you should ever try migrating the website to a different system, firstly due to the URL being tied to index.php?id=1, if your new system is .NET based you will need to setup rewrite for index.php. Secondly your if the system you are migrating to already had content which are using the id's then you would end up with all your content being mapped to new id's and you would have to start thinking of a suitable solution to redirect previous traffic to the new URLs.
In the context of a Blog, It simply won't act as a breadcrumb for your website. If I post an article called The Next iPhone in category Apple and the URL looked like this http://www.mysite.com/blog.php?category=1&id=2. Based on the URL alone the user wouldn't be able to tell the category or what the post was about. If the URL was written a bit more friendly http://www.mysite.com/blog/apple/the-next-iphone the user would be able to tell more.
They are just plain old ugly, they are user unfriendly and hard to remember. Where as their opposite look prettier, friendlier and easier to remember.