There's another con: they can be unreliable, make mistakes and mess up everything!
The cost is really not the problem - I'm happy to pay a commission for a job well done. The problem is finding such a sourcing agent that is RELIABLE and TRUSTWORTHY. I'm sure there are such services out there but go and find them.
Two times I used such a service (a reputable service at that time), they mixed up everything and cause massive expenses at my end to sort it out.
Then one of my clients used one well-known sourcing company in China, run by expat etc. and basically they screwed up him totally! The goods were delayed by many months and when they finally showed up, half of the order was missing.
That's my experience with sourcing agents. I don't use them because another middleman creates a higher risk of things going wrong. I'm, not saying that everyone is like that, absolutely not! But people should be very careful when choosing a sourcing agent to work with.
I would use such a service IF I knew someone who is 100% legitimate, genuine and trustworthy.