There is no down side to it.
Will you still be convinced of that once their 'main' purpose is enacted - to enable the power companies to micro-monitor your usage, nag warn you to change your energy use habits and, if they deem it necessary, cut or restrict power to your home/individual appliance?
I'm guessing that this is one of the ideals of smart/digital motorways before too long, once we all have to have electric (interconnected) cars: it won't just measure the general traffic flow and tell all of the drivers to drop to 50mph on the overhead gantry - why would it if it has the ability to ping a message direct to your car, give you a very short time to obey and then, if you don't, remotely activate your brakes for you (and/or generate an automated speeding fine)?
For all you know, they might also seek to monetise your usage data by selling your details to other companies - probably claim it's 'legitimate interest' and that, by getting a SM and using electricity in your home, you've already consented to them telling Currys that your washing machine's very poor efficiency suggests that it's an older model, so they can keep writing to/emailing/calling you telling you that it's your social duty to buy a new, much more efficient one to replace it.... and maybe even report you to an energy-saving equivalent of social services after a while if you have 'trouble' understanding and doing this.
Just called them and a rather stern lady called Jean told me that of course this is not my actual meter reading and to just ignore it.
That's just rubbish, though. If it gives you an absurd amount - thousands of pounds a day - it's obvious you need to avoid it, but what if it were on average out by, say, 5%, 10%, 20% (whether under or over)? How would you know? If you can't ALWAYS trust it, then you can't EVER trust it.