There are a few situations in which it might be a good idea to use plastic instead of glass in furniture design:

1. Durability: Plastic is generally more durable than glass and is less prone to breaking or shattering. This can be especially important in furniture designs that are intended for use in high-traffic areas or by children, where the risk of accidents or damage is higher.

2. Weight: Plastic is generally lighter than glass, which can be beneficial in certain furniture designs. For example, if a piece of furniture needs to be easily movable or portable, using plastic instead of glass can help to reduce its weight and make it easier to handle.

3. Safety: In some cases, plastic may be safer to use than glass in furniture design. For example, in furniture designs that are intended for use by young children or in areas where there is a risk of accidental breakage, using plastic instead of glass can help to reduce the risk of injury.

4. Cost: Plastic is generally less expensive than glass, which can be a factor in furniture design. If cost is a concern, using plastic instead of glass can help to keep costs down.

Overall, whether to use plastic or glass in furniture design depends on the specific needs and requirements of the design, as well as the intended use of the furniture. Both materials have their own unique properties and can be used effectively in different situations.