You can probably do about 60% of what Silver Efex Pro does in Lightroom and Photoshop without too much difficulty. Where the Nik tool excels is in that other 40%, allowing you to do extremely fine manipulation of brightness and structure by zone (highlight, midtone, shadow), and also thru the incredibly useful Fine Structure and Soft Contrast sliders. You can surely take the time to make those adjustments using multiple layers and masks, but you're talking a significant amount of time compared to what you'll get in Silver Efex. Then there are the added benefits of predefined grain patterns and level curves to replicate film types, the toning section, burning and framing sections.
The differences gained with that 40% can be subtle, but it's also what often makes for the difference between a great image and a "Wow!!" image.