This one appears in nearly every version of D&D that I can think of, yet I have rarely seen it used. Maybe after druids became more of a thing, but as a clerical spell? And a relatively high level one at that (4th)? Not so much. It's not that SwP doesn't have its uses, but it takes up a slot that could be used by things like Neutralize Poison or Cure Serious Wounds. The good news for the spell is that since it's clerical, the caster can trade out for a day if need be without it taking up a fixed spot in a spell book.
So what exactly does the spell do (or not do)? It has a range of 30' (yards in wilderness) and a duration of 3 turns. It "gives the cleric the power to talk to plants and request simple favors of them." One example the description lists is undergrowth bending out the way to allow a clearer path through. It also allows communication with plantlike creatures (eg Treants).
An interesting caveat is that while a request is automatically received favorably by the plants, they must be able to perform and understand what the cleric is asking. Since the spell's operative verb is "speak" it's fairly obvious that covers the comprehension part of things. I imagine "understand" in this context has more to do with the complexity of the request.
Beyond the undergrowth example, or translating Treant-ish, I could see this spell being handy for things like quieting threats from shriekers or yellow mold. I know I've already written about fungi vs. plants on this blog, but for simplicity in gameplay let's just consider them equivalent. (If there were a "Speak with Mushroom" spell I might have a different opinion.) One could also use it to more effectively camouflage a party to hide from pursuit or to stage an ambush. It could also be used to wipe out a trail to prevent being tracked.
I have to say the spell contains a lot of potential for creative uses. I just wish it was lower level to make it less unlikely to see play. The Ring of Plant Control offers similar options to the PCs, but is much more powerful in many ways. Maybe a scroll or potion to introduce the spell into a campaign instead?