Insurance underwriter here. I'm on the side of don't buy the umbrella unless you've got a compelling reason.
Umbrella premiums are cheap because the loss ratios are low. Yeah, we'll pay multi-million-dollar losses, but overall the ratio of money out to money in is in our favor. Umbrellas are one of the most profitable lines of insurance.
VirginiaBob is absolutely right that any attorney will know what they can potentially get. That information comes out in discovery. My personal view is that having an umbrella makes you a bigger target.
If you think you have enough assets to justify an umbrella, do some more analysis to find out what could *actually* be taken in the event of a suit. Most states have a homestead exemption to protect your primary residence; find out what the terms are for you. Under ERISA, 401ks and defined benefit plans are protected from all forms of creditor judgement. IRA protection varies by state but can be unlimited. (This can also be very low. My state is something like $75k, indexed for inflation.)
Reasons to get an umbrella (IMHO): Dogs, trampolines, vacant land, youthful drivers, professions with a history of being held personally liable for professional torts.
If you really feel that you have enough exposure to justify an umbrella, or you're willing to pay for it to sleep better at night, check into raising your deductibles at the same time. Going from $500 to $5,000 on your homeowners policy would easily pay for the umbrella. And you can afford a high deductible with all those assets you're protecting.
Final thought: if you ever outsource housework or home repairs, get proof of insurance.