1. LLCs are partnership-like entities governed by an operating agreement and every owner (member) has to sign and agree to the terms of such agreement, meaning also that every owner (employee) gets to see in intimate detail who owns what within the company, who has what management authority, who has invested what, and the like. Bad for the company.
2. LLCs have no counterpart to ISO-like grants, meaning that everyone who exercises options can (normally) effectively do so only at the time of a liquidity event because he would otherwise have to pay an immediate tax on the "spread" (difference between exercise price and fair value of equity interest acquired) as of the date of exercise and would thereafter hold an illiquid investment for what could be many years. Bad for the employee.
3. Apart from the limitations of LLCs themselves, it becomes difficult to administer an option plan when the value of a company is already high because it costs too much for employees to buy into it. For example, if a company is already worth $10M, the exercise price of options needed to buy .001% of it would be $10K. How many employees want to part with real dollars for the hope of a speculative return way down the road, especially if the company itself says it has no plans to move toward a liquidity event any time soon?
Hence, the alternative plan adopted here makes sense. It does not represent any form of true equity interest but is an employee bonus plan. This means you lose the benefits if you leave employment. It also means you get, at best, employee compensation that is subject to tax at ordinary income tax rates as well as to all employment taxes (though no tax is due until the bonus is paid). It also means that the interest is not transferable. It is basically a special reward granted to those who stick it out over the years until the company gets to a liquidity event. It is not perfect but, given the problems noted above, it is actually a pretty good solution to the problem of how to afford special incentives to key employees whom you want to add value to a growing but mature company operating in LLC form.