Gluing Table Aprons to Plywood Table Top
Glue is rarely a bad thing to add to a (permanent) joint. Pocket hole screws are plenty strong on their own for most typical applications, but adding glue only makes the joint stronger. Plywood is very strong and very stable (two big reasons for using it), so there's no concern related to that. 

One potential problem that may develop as a result of using glue is getting the joint to line up nicely, especially while screwing in the pocket screws. Glue acts as a lubricant here, allowing the two pieces to slide around quite a bit during clamping. If the two pieces are not clamped very securely, they will move when driving in the screws. If you can come up with some kind of registration / alignment system it will make the whole process much easier. Use biscuits, dominoes, rabbets (UK: rebates) (even a 1/16" deep rabbet would aid alignment), whatever to keep the apron aligned on the tabletop.