Maybe I'm missing something. If I have a 2 man guard squad and a 6 man con squad I'm either going to have to retreat the guards or I'm going to have to merge the cons into the guards and retreat the cons instead. Either way I will have at least 4 con models worth of durability on the field. The only difference would be that one option seems them equipped with guard rifles and providing vet to a guards squad while the other sees them with con rifles and providing vet to a con squad. You are right that in the long run my guards will be a bit less durable for a while but I think I would rather take the better immediate field presence and just reinforce the guards properly the next time they have to retreat during an engagement.
Well, you're describing the last resort situation.
I have on mind constant reinforcement, say you lost 1 men after a shootout, you merge it, now you've got tad weaker squad, on the long run and especially in late game, where due to LMGs rec acc matters a LOT, you'll be making your opponent a favor while trying to keep your squads on field without M5.