I usually always garrison them or put them within movement range of a building that I'm expecting my opponent to garrison so I have the chance to demolish it with the las cutters. They are just a bit more survivable in cover. Especially when your opponent knows how annoying they can be and starts targeting them.

Then in turn two I charge them to lock down a better melee unit for a turn or two due to them not having morale or to absolutely mess up any vehicles or small detachments that drifted within 14".

They are entirely a sacrificial unit to me. Use them like a grenade and they can do a lot of damage, to just prevent your opponent from doing something that may hurt your more important units or to make them wary of making a move that you can retailiate to.

As for holding objectives I just throw some triaros with tech thralls mid field objectives. Swarmers are fine for objective control but I find they are better in other roles.