There have been three previous mass amnesty grantings by the home office. Check out http://www.migrationwatchuk.org/briefingpapers/policy/asylum_amnesty.asp?search=amnesty where the details and numbers are listed.
The reason given was to "clear the backlog" of applications. As there are approximately 750,000 illegal immigrants in the UK , based on numbers of failed asylum seekers who have not been deported, an open amnesty would simply encourage an even faster rate of illegal immigratation. Those who see this as a benefit probably don't live in the South East of England or pay the taxes required to support the welfare payments and new infrastructure development which this rate of settlement will generate.