Honestly, I had not registered my earlier systematic reviews with PROSPERO. However my last two rounds of reviewer comments have specifically asked if my protocols were registered. I have started registering my protocols to make the review process a little easier, and would recommend my students to do this also.
Yes it is extremely helpful, should you desire to published your systematic reviews in a high impact journals with good audience then PROSPERO registration is a must: quoting from my own experience; all the systematic reviews that we have submitted to BMJ requires PROSPERO registration number when submitting for publication. Having said that, PROSPERO registration will also help in preventing duplication of your work, hope this helps.