Bills being debated in Parliament may eventually form the basis of UK law and as such they require careful attention, if only a handful of MP's are present at a debate then how can they be said to be paying this careful attention we require of them.
Yes this will slow down the process of creating new laws, however the process of creating laws should strive for accuracy and not speed.
Further if an MP does not pay attention to a bill by being present at it's debates they should not be allowed to wander in at the last minute and vote as instructed by their party whip.
Yes this idea will bring an end to the party whip, but is that such a bad thing ?
For example with the Digital Economy Act, during the debates there were times when under 20 MP's were present, yet when it came time to vote the party whip ensured a sufficient number to cast their vote as they were told.