"Piggyback" is sort of a slang term with no single definition. Some Nurses refer to a y-site hookup as a piggyback, others call a secondary a piggyback.
Maybe in your practice, this is true. However, I was taught both in nursing school and have learned in practice that piggybacking and Y-siting are specific, distinct procedures.
From Intravenous Infusion Therapy for Nurses, Principles and Practice
by Dianne L. Josephson
"The secondary infusion is initiated after the primary infusion is in progress. It is the most common method to administer IV medications concurrently with the primary infusion. It is coupled to the primary infusion line at the first injection port
below the check valve. The piggyback
is able to function concurrently with a primary infusion only when it is suspended higher than the primary line (which must have a back-check valve). By opening the clamp on the secondary line, the primary infusion temporarily stops flowing. When the piggyback infusion is complete
and the infusate in its tubing falls below the level of the primary line drip chamber, the back-check valve opens and the primary infusion resumes
"The Y connector
is a device that provides an access route for two IV fluids to infuse at the same time."
Also, I agree that it's not good nursing practice to start unnecessary IVs, as it exposes the patient to further risks. However, policy where I have worked has always been that if a patient has a continuous drip, whether Lasix, heparin, NTG, Cardizem, or whatever, that medication needs to have a dedicated line that is not used for anything else. This means if the patient is receiving other medications intravenously, a second peripheral is indicated, and a central line should be considered.