The subreapeater is not even for people who lack WP (Rifles), since in order to brace it, you have to open the folding stock, which makes it require Rifle proficiency to use. And if you want unbraced autofire, just buy a blaster rifle or carbine which are more powerful and use them with the stock folded.

As far as I can determine, the only use of a subrepeater is when a character heavily invested in Pistols needs to make braced autofire attacks. Which is a pair of sets with no overlap IME.