First of, in the UK, under the CRB (yes I know it's been replaced) legislation, a lot of offences are effectively lifelong anyway. So we're already in a world where the concept of spent convictions is hazy.
The real problem is what is the purpose of the criminal justice system ? If it's just to punish, and protect. Fine. However, I subscribe to the rather quaint notion that it's also to rehabilitate. That is to turn an offender into an upright citizen. If you foster a system where a conviction for (say) graffiti effectively marks you as a criminal for the rest of your life, and you are unable to ever get a job, then where do you think society will go ? If 50% of the population has a criminal record for something, sometime, then you devalue the concept of a criminal record. Much as the concept of speeding has been devalued. So many people have at least 3 points on their licenses, most insurers ignore it. It's meaningless. It's the same thing with the 3,000+ criminal acts the last Labour regime bought in.
It's axiomatic, of course that these pettifogging convictions will be no bar to the rich and powerful.
The idea of "spent" convictions is that they allow a person a chance to go straight. 10 years, no trouble, they are considered rehabilitated.