my main concern about servitors is that they fall like flees. super fragile. a simple weapon kill them all and they can easily targeted. So, is it worth?
Considering that the maximum number of servitors (with rule of 3) just costs 60p, I'd say it is worth the investment. Repair just one destroyer, and they pay back every point instantly.
Per rules, they can be targeted like everything else, but considering the theoretical range from target to servitors (36" kataphron weapons range, 6" from kataphron to dominus, 2" dominus base, 6" from dominus to servitors = 50"), only few weapons remain to take down the servitors, unless IG mortars or something similar are present in abundance. Threat management would be rather easy - have any other semi-useful infantry unit on the field, and those will be targeted instead. Remember - they're as resilient as skitarii (minus the 6++), just cheaper. There's worse for 5ppm out there (hello, kroot), and they can even serve as objective grabbers, deep strike blockers or CC speedbumps if necessary - better to entirely protect the kataphron from a threat, sacrificing the servitors, than getting one kataphron back later.
Also, the repair happens at the end of the movement phase - the servitors can stay back/behind cover, and only move into dominus range when a repair becomes necessary. And advance as part of that. And this edition's Mindlock rule doesn't prevent them from acting on a bad roll, meaning they can probably sit in cover/out of range, playing unappealing target, and run to the dominus when spare parts are requested.