Re: Nice idea..
How easy to type, how hard to implement in practice.
The idea behind UC was kicking around for YEARS in DWP before IDS came in. A smaller attempt was made to bring benefit systems together in the mid 2000s which was scrapped as it quickly ran into the sand.
There are two reasons the benefits system is complex - one is the incremental nature of it, with things being layered on top of each other for years and years until we end up where we are now.
The other is that life is stupidly, incredibly, ridiculously, unbelievably complex, and the benefits system has to take account of that as the impact of ignoring it can have profound consequences for people.
Lets take an example - I'm a bank and a customer wants to open an account and give his 3 wives (he has a polygamous marriage from his time living in Qatar) access. A bank may look at that use case and say "screw it, too costly to implement the functionality, hardly anyone needs that so I won't build the functionality in to the system and if he doesn't like it I don't care, he can naff off to another bank"
The DWP can't say that, as if he can't claim benefits he and his wives and his kids will starve (lets ignore the arguments over whether or not he should get benefits). In the UK polygamous marriage is not legal, so which marriage do we recognise (the first one, as the rules currently go - yes this is a real example I've worked on in the past) ? What do the other two women with kids do about their benefit claims? They all live in the same house, so how can we work the rules to ensure they have enough to live on, but don't get more than their fair share and take the piss.
If you think that is a ridiculous use case, then you clearly have never worked on social security or tax systems. There are many many many examples like this - my favourite is the guy who changed his name to his national insurance number which caused a re-write in a bunch of modules as it broke some validation rules designed to weed out mistypes.
As my boss used to say, when your IT system has to deal with the entire working population every month (for example with national insurance), then one in a million situations happen at least once a day.