I assume you are talking about advantages and disadvantages when compared to a traditional wedge gate valve.
The two main advantages of parallel slide gate for steam service are:
1. A parallel slide gate valve is much less likely to get stuck in the closed position. This is because the gate is not torqued into the seats like a wedge gate. For steam generators that frequently start up and shutdown this is a major issue.
2. A parallel slide gate valve generally provides better shut off than a wedge gate.
The main disadvantage of the parallel slide gate valve is that it's more expensive than a wedge gate.
There may be a few other advantages and disavantages but I believe these are the main ones.
Parallel slide gate valves are used at the power generating stations. The advantages over gate valves is lower friction, cannot be damaged for overtightening, and better sealing due to the flexubility built in the design. The manufacuring does not require matching gate to the body. I have used these with great succes and no maintenance after 8 years so far. Check with Velan Valves for more info.
