Many GNOME developers seem to regard launchers on the desktop as clutter. GNOME 3 does not permit them at all, while Unity easily allows templates and documents, but requires users to go through the file manager if they want to add application launchers.
Yet the desktop is the quickest place to access commonly used applications, placing them only one click or double-click away. By allowing app launchers on the desktop, GNOME 2 automatically appeals to a large section of users that other GNOME desktops do not.
GNOME 2 and its successors sometimes crashed on me, requiring a restart of X Window to restore the desktop. But I can't remember a crash that permanently brought the desktop down and required laborious recovery, such as creating a new account and transferring personal files to it. I can't say the same of KDE, GNOME 3, or Unity.
In the past, GNOME 2 was sometimes dismissed as bloated, with its last releases realistically requiring half a gigabyte of RAM. Compare to a window manager or a desktop like Lubuntu, that figure seems high. But, compared today to KDE, GNOME 3 and Unity, all of which require a gigabyte of RAM for decent performance, it seems economical. Not only will it run better on older machines, but on modern machines it appears far more responsive.
GNOME 2 was designed to accommodate users with different work habits. Users could make launchers on the panel or desktop part of their workflow, or open apps entirely from the menu. They could use virtual workspaces, or ignore them. Essentially, it could be customized for the user, rather than forcing users to work the way its developers expected.
For all its experimental features, KDE still has this flexibility. However, both GNOME 3 and Unity are built on the assumptions that there is one way to use them efficiently, and that users should adjust to the built-in expectations. If these assumptions fit the way you work, you are unlikely to have any problem with them. But the trouble is, many people find the assumptions awkward and distracting. Such people are likely to feel less restrained in GNOME 2.
Reading comments on various forums, I am almost tempted to add a tenth reason for using GNOME 2 clones. The design of GNOME 3 and the GNOME project's general ignoring of user complaints has apparently caused considerable anger -- far more than even KDE 4.0's release did. Unity has received similar complaints. To many GNOME users, these modern desktops continue to feel like a betrayal by developers.
To those who continue to feel this way, what better way to register your anger than to revert to an earlier release series, or to activate a group of extensions like Linux Mint's Cinnamon that converts GNOME 3 back to GNOME 2? But this reasoning, if it exists, is probably unconscious.
A GNOME 2 clone is never going to be the most innovative of desktops. To many eyes, it will look antiquated. Yet it gets the job -- any job -- done without any fuss, and respects users' choice, and that combination is rare enough in modern GNOME that solutions like Mate and Cinnamon are likely to have a wide appeal for years to come.