If there is no inventory at the beginning, or if it is not detailed enough, doesn't relate to the specific tenancy being checked out, it is impossible to establish the exact condition at the outset of the tenancy.
If you are unable to establish a benchmark to measure against, there can be no comparison.
My previous LL checked me in herself with an inventory that related not to me, not to the previous tenant, but the one before that. She had simply written on it and scribbled stuff and photocopied it. It was 4 years old by the time I checked in.
When I left she tried to take over £450 from my deposit. it went to TDS and they kicked it ALL out due to the inability of the LL to establish condition.
I have since become an inventory clerk myself. Sometimes I come across 'inventories' prepared by the LL. 2 pages worth for a 3bed property? 
Basically as long as the tenants have not taken walls down, or sawn doors in half, burnt the kitchen for firewood, their deposit is pretty much safe.
My job when checking tenants out is to compare the original inventory with the condition of the property in front of me, and judging on the number of occupants, the ages and the length of the tenancy.
If I can not know for sure what condition it was to start with, how can I say with complete confidence that this was due to the negligence/neglect of the tenant?
The more detailed the inventory, the more accurate it is. The more accurate, the easier it is for ALL to see the difference between before and after.
We do have many tenants who DO genuinely improve the condition of the property, through cleaning what wasn't marked as clean when they moved in, or who repaint walls during the tenancy, some sort out the garden, replace carpets. Some go mad and spend tons on redecorating.