Given all the hacking scandals swirling in the air, I think it was good of the Secret Service to hold on to his phone. I hope they do it more often going forward. That will save the entire world from Trump's terrible tweets. Besides, I don't think it's up to us to question what the Secret Service does.
I can think of reasons why a president-elect should hand over his phone before the inauguration, not least of which being that he should not need it during that time. It is good for the people to know that he is focusing his attention on the task at hand. If the Secret Service feels that there is a security risk, they should be allowed to do whatever needs to be done to mitigate that risk. The president needs to allow them to do their job so that he can do his without distraction or risk of harm.
Most prominent politicians let their staff manage their communications, but Donald Trump is famous for his unfiltered, direct communication with the outside world. And on the campaign trail, Trump’s Android smartphone was his most important communication tool. His most controversial tweets consistently came from his personal phone, and he spent a lot of time talking to people on it too. But as he becomes president today, Trump is being forced to trade in his trusty Android phone for “a secure, encrypted device approved by the Secret Service. If Trump had continued to use his old Android phone, it could have put him at greater risk of attack from foreign governments and others wanting to intercept his communications.
Trump has proven that he is incapable of accepting criticism of any kind and his tweeting has gotten out of hand and can be dangerous now that he is President. Perhaps restricting access to a personal phone is necessary to keep him focused on the job at hand: working for the American people.