The German dive operation we've used for the past two years at the Red Sea in Egypt (Extra Divers/Makadi Bay) seems to balance this just right for boat dives.
Their rule (Egyptian law?) is that divers with less than 20 total dives have to dive with a "guide"... usually the boat's divemaster. With a 5 Euro surcharge. There are usually several divers on each boat that fit that requirement. If they don't already have buddies, the divemaster organizes them into two-person buddy teams, with the divemaster either diving solo or with a buddy; and off they go.
How good the guide is obviously varies from guide to guide. Sometimes they just lead the way, watching out over their charges. Most times they actively seek and point out interesting stuff that divers, experienced or not, would probably miss otherwise.
Divers with more than 20 dives can go off on their own; subject to a one-hour maximum guideline, and a max depth according to certification. They usually get dropped into the water first.
If more-experienced divers want to tag along with the guided divers, that's usually fine too (like in our case, parents with children); the guide/divemaster obviously asks that they stay behind the guided divers.
--Marek