Unlike real estate, we don't have a large buy-in cost, so we can rent out very very cheaply; far cheaper than just about any other form of advertising. If you have a generic descriptive domain that gets 30 or so type ins per month, why not lease that domain to a prospective buyer for say £5 per month? Less than 20p per click. On a name that has a £3 per year carry cost, that's a great return. You get to keep the domain and benefit from its capital growth. Hopefully, the key to finding 'domain tennants' will be in not being greedy with the rental fees. Anything over 25p per month is profit. Not only that, but if you can show a portfolio of leased names with a regular rental income, you have then got a product worth selling, either in part or whole.
That's an interesting way of looking at things. However, the "damage" somebody can cause to a domain name, for example by using it to spam, by promoting it inappropriately, or indeed even by associating it with a very specific product, company or industry for a couple of years and then suddenly dropping the lease (forcing the domain name to be redirected elsewhere) is potentially huge.
Legal protections could be put in place to govern a lot of the above, but there's no margin to draw up and enforce the type of contracts that would be required if you're talking about pennies or pounds a month.
Not to mention the "credibility factor" in that somebody in a management/decision role at a company of any size simply won't be willing to take the time to get educated about the benefits of a service that costs a few pounds a month - they're "on the clock" on company time, and that time's far too valuable to spend it grappling with something they're initially going to be completely unfamiliar with.
That's not to say leasing wouldn't work - it is probably a very exciting business model, if done right and with the appropriate legal and other groundwork up front - but it's going to need to be priced at a more "credible" level of probably no less than £50-100 a month to get any traction at all.
Expanding on the above, there's potentially a new service opportunity for a turnkey, credit card billed domain leasing service, where the lease-taker can set up the arrangement with a few clicks and pay a deposit plus the first quarter's lease up front, giving them instant access to certain DNS settings for the domain name. Then (say) 15 days before the end of each quarter, they will be rebilled for the following quarter, allowing them to maintain their control over the domain.
Taking this one step further, the leasing provider could ultimately take the domain name "into trust" (with all appropriate/necessary safeguards) thus reassuring the lease-holder that the owner of the name literally can't run away with it/sell it from under them, thus disrupting a marketing push.