I mainly think publication in PROSPERO is necessary for reducing the risk for introducing risk of bias in your systematic review (quality issue). Moreover, it is may be of importance to ensure that resources are well spent. The downside is of course if you scoped on your publication. Not always simple.

Assuming the good will of fellow researchers, I believe such a situation occurs due to lack of awareness among researchers of the benefits and even existence of PROSPERO database that is indeed an excellent initiative from the Centre for Reviews and Dissemination at University of York, UK!

Although, there is a noted and continuous increase in its usability.

A recently published review entitled, ' Registration of systematic reviews in PROSPERO: 30,000 records and counting', (Link: Article Registration of systematic reviews in PROSPERO: 30,000 recor...

stated the following conclusion:-

'Registration of SRs in PROSPERO increased rapidly between 2011 and 2017, thus benefiting users of health evidence who want to know about ongoing SRs. Further work is needed to explore how closely published SRs adhere to the planned methods, whether greater pre-specification of outcomes prevents selective inclusion and reporting of study results, and whether registered SRs address necessary questions (Page et al 2018). '

I believe that searching the protocols in the PROSPERO database is an essential step any researcher should conduct before writing his/her own systematic review protocol and if the review of the targeted topic is ongoing, the researcher can simply contact the research group and offer his/her contributions or if it's already completed concentrate on contributing to its later update.

I'm totally agree with you. There are lots of papers and can not be controlled even with registration. So researchers should extand their search on the topic if there is any other group, registered or published articles.