The songs are your intellectual property, so by that token they're protected whether you register with the PRS or not. If Ed Sheeran decided to nick a chorus, you'd be able to prove that he'd stolen it from you and take him to court for it.
If you feel this could happen, then sign up for the MU who offer legal protection.
When it comes to royalties, PRS can claim these for you if your songs get played. To generate anything like a few pence, they'll need to be played thousands of times. Having had these conversations before in bands (once when Vodafone nearly used one of our songs for an advert*) the classic response to this is, 0 spilt 10 ways is still 0.
Personally, I'd only consider registering with the PRS if there's the possibility your song will receive airplay on the radio, TV or be featured in an advert or whatever. Until that point, I'd consider it a generous gift to the universe.
(*Vodafone didn't use the song and the German label we were signed to sent us back thousands of unsold copies of the album which still remain festering in locations throughout the UK.)