Firing Anti-parallel Thyristors with continuously firing pulse-trains
It is safe to use pulse train triggering. Actually pulse train triggering is the only way to handle inductive loads with thyristors or triacs. There is no danger at all. If you are not using pulse train triggering a DC component could flow in such situations.
Very nice explanation about pulse triggering can be found in:
http://www.datelec.fr/secteur/ST%20AN308.pdf

The pulse triggering as indicated in the drawing of OP has a high frequency in respect to the mains frequency and is required in cases of inductive loads triggering failure due to Back EMF from DC motors.