I second Ripley's earlier point. Ask the end-user prospect if they'd like your domain name "for use with" their site. Stay away from any negative language at all, such as that implying their website's URL is obfuscated, or even that your domain might be a more memorable alternative. Remember that when your end-user candidates read your pitches and hear your voice on the phone, the left of their brains are assessing the domain's quality/relevance while the right half is assessing YOU. It's incumbent on you to sound pleasant and, more importantly, comfortable. Do everything in your power to help the end-user feel important. I've found the statement
"I'm offering you [domain].com for use with your [domain].net because I checked out x, y, and z on your site, which really touched me because of [relevant point from my background], and I'd love to help add value to your organization."
extraordinarily effective, more so than
"I'm offering you [domain].com for use with your [domain].net because it will [solidify your brand, prove more typo-proof, etc.]"
Give it a shot one time, particularly if you have your end-user client on the phone. You'll be pleasantly surprised at the results.