When should a plastic volumetric flask be used instead of a more accurate glass flask?

Typically plastic volumetric flasks are used when performing trace analysis and analytes maybe absorbed OR desorbed from the glass.

An examples is estimating [Na+]  in aqueous solutions by a a process like ion-chromatography--if you acid wash your glass volumetric flasks then Na+ in the solution can be absorbed by the glass and then if you base wash your glass volumetric flask, then you may have excess Na+ leaching out into your solution.  So to avoid leaching out or absorption of Na+ ions, you would want to use plastic fvolumetric flasks.

The other instance you would use plastic vessels is measuring out solutions of strong bases like KOH and NaOH--glass slowly dissolves in solutions of bases--you will get potassium or sodium silicate in the end and glass is permanently etched by strong bases--so in these instances you would want to use plastic vessels

As regards acids--the only problem you would have is with HF--glass would be etched/dissolved to form H2SiF6 and related soluble compounds

So in reality All the answers are right in the proper instances.