You should definitely be able to use SW:P in a raid. If your guild would like more concrete evidence, I would recommend playing around with Guybrush's Warlock spreadsheet:
https://forum.nostalrius.org/viewtopic.php?f=40&t=12421Not only can you toggle Shadow Weaving on and off to determine how much dps Warlocks gain from you, but you can also see how much dps Corruption actually provides to them. Corruption is only a 30-40 dps boost (for both SM/Ruin and DS/Ruin), where as SW:P is 150-175 dps for a Spriest.
The last time I played around with Shadow Weaving on the spreadsheet, I found that it provides ~106 dps to each Warlock in the raid. Spriests cap out around 525 dps and Warlock's cap out around 800, meaning you only need 3 Warlock's in the raid to make a Spriest worth having.
Note, all my numbers are based on BiS gear and full consumables, but no world buffs. If you toggle world buffs on, Warlock dps shoots up (because of the ridiculous amount of crit), which also shoots up the value of the 1 Spriest in the raid.