Well, as a specialist wizard/loremaster with the Use Magic Device skill, my PC can try to cast cleric scrolls with Use Magic Device, so why not prohibited spells?
The Use Magic Device skill is an entirely separate mechanic for getting magic done. If the PC is going to dedicate the time and effort to learn this other way, why forbid them the opportunities of this path?
It ain't easy. You need some serious investments of points to start working with high-level spells. Unless, of course, your DM is lame enough to allow one of those +20 to Use Magic Device items that everyone talks about.