The police want to check that accusers did not have an undisclosed relationship with the accused. The most high profile case was one where the accuser was later shown to have pestered the accused for sex after the alleged incident. I cannot think of any reason why he did not say this in his defence, and provide his own mobile as evidence.
The police case seems to be that there might be evidence on the accuser's mobile that would undermine the rape claim. This argument is riddled with loopholes and judgments. Evidence of sexual promiscuity, for example, does not make rape any less of an offence.
Rape is as much about power as it is about sex. By choosing to deprive the accuser of their mobile without sufficient grounds is tantamount to continuing the abuse. There is so much personal information on a smartphone, and the police would need to comb through all of it to determine if any of it is relevant. I would not want to give a stranger access to messages to and from my family members at times of distress, for instance. The police are not above data breaches or unauthorised access to information.
Why are the police so keen to undermine rape allegations but not others? If I am accused of stealing a TV from someone's living room at dead of night, I would expect them to prove they owned the item in the first place, and that they had not sold it in an insurance scam. Our legal system is set up to reduce risk of unjust convictions, so the defence need to be able to confirm that an offence actually took place, but there is no proposal to seize mobiles from those claiming to have been burgled.