"Is it really necessary to download and run the flash uninstaller before getting the new version"
no not normally, unless something goes wrong with the upgrade.
"It always seems to be a great palaver when adobe issues an update or another version of flash?"
Flash (like a lot of Adobe applications) has had a huge amount of bugs and security issues in it, the latest versions address some of these and also upgrade some of the features availble.
"If I do nothing and leave the present version on computer,will it really make a difference?"
You will be leaving your system open to having the above mentioned bugs exploited and although most flash sites will continue to work you will find, over time, an increasing amount will make use of the features in the new versions of the players and will no longer work.