Chetan, there is no reason really not to modify the base page types. In fact, you can build a whole site around your own custom page types if you want without using any of Kentico's base page types.
The page types we modify the most often are page, blog, blog month, blog post and event booking. All of these are base page types with built in functionality we've modified for over 9 years without issue. Once you clone them you lose that out of the box functionality. The OP is looking for the Event Booking functionality but needs to add some additional fields and functionality so why recreate it?