I always use my Cons as backup of my Guards, when squad number becomes low, I use merge.
It is considered as reinforcing of course.
I do the same with Penals (not a fan of Guards, didn't tested them after PTRS buff).
I see that we have different understanding of word "reinforcement".
I understand it as "getting more soldiers" not "more soldiers on frontline while one or more squads will be running to the base and back instead of actually fighting".
And, M5? FHQ?
Anyway I predict he would said how "trash" they are.
They are no trash at all ofc. But FHQ is doctrinal and immoblie and M5 locked behind T3 which is optional choice and comes quite late unlike 251.
You can't deny that in terms of field reinforcing (paying MP to bring back lost models) 251 is the best.
Ambulance is very close as it can be placed near closest territory border and provides healing.