In early days every peripheral needed a driver to be installed.Today,thanks to USB peripheral devices we don't need any drivers for them and they are not even dependent on the port. But haven't this USB peripheral device reduced the security of the peripherals? I mean it would be easy enough for someone out there to have access to my peripheral device if I have mistakenly left any door open for him to access my PC. Is it dangerous this way?
It is not really a security risk. Your question assumes that someone already has access to your PC. If a malicious user has access to your computer, then you've already lost.
With physical access someone could remove your hard drive and read any unencrypted data, install a PS/2 key logger, use a floppy or CD with auto-run, put on a network tap, or just simply take your computer. There are some many things that a malicious user could do with physical access that plugging a USB device device doesn't make the situation worse.