You've hit on a point of common discussion and some confusion regarding handle ties and multiwire branch circuits (MWBCs). Let's break down the rationale and address your points.

Understanding the Purpose of Handle Ties/Common Trip Breakers

The primary goal of handle ties (or, more accurately, common trip breakers) on MWBCs is safety during troubleshooting and maintenance. The Code intends to ensure that when someone works on a circuit leg of an MWBC, they disconnect all energized conductors supplying the circuit, preventing accidental contact with unexpected voltage.

Addressing Your Points

1.

"Nec does not say the handle ties must be mechanically connected so why require special breakers at all?" You're right; the NEC often implies but doesn't explicitly state "mechanically connected" for handle ties in 240.15(B)(2). This ambiguity is a point of contention. Here's why this distinction matters: Handle Ties (Simple)

These should mechanically connect the handles so that turning one breaker off should turn the other off. However, there is no standard saying that the handle ties provided with single pole breakers (to make a multi-pole breaker) will reliably trip the adjacent breaker in an overcurrent situation.

Common Trip Breakers

These breakers are internally linked so that an overcurrent condition on any pole will trip all poles simultaneously. This provides a far more reliable guarantee of all conductors being de-energized.2.

"However if the ring or tie is available but not installed they still serve as single pole breakers." Correct. If the handle ties are available but not installed, they function as single-pole breakers. This defeats the purpose of 240.15(B).
3. "if the intent is to disconnect all ungrounded conductors then why not just us 2 pole breakers?" In practice, using 2-pole breakers (or 3-pole for 3-phase MWBCs) is the best and safest approach. Many experienced electricians always use a multi-pole breaker for MWBCs for this reason. Cost

Two pole breakers cost more than two single pole breakers and a handle tie.4.

"2 same for single phase circuits line to line loads" For 240V line-to-line single-phase circuits, a 2-pole breaker is always required. This is because 240.15(B) specifically requires that the circuit breaker open all ungrounded conductors simultaneously. The exceptions in 240.15(B) generally pertain to MWBCs with line-to-neutral loads.5. "3 same for 3 and 2 phase systems with ground" For 3-phase (with or without a neutral/grounded conductor) and 2-phase systems, a 3-pole or 2-pole breaker respectively is always required to disconnect all ungrounded conductors.

Why the NEC Isn't Explicitly More Restrictive (and why the trend is changing) Evolution of the Code

The NEC changes over time. Early iterations of the code were less specific on this issue. As incidents and understanding of safety hazards have increased, the code has been gradually updated to become stricter.

Cost Considerations

Making multi-pole breakers mandatory for all MWBCs (line-to-neutral) could increase installation costs, a factor always considered during code revisions. However, safety concerns are increasingly overriding this argument.

Industry Pushback

Some manufacturers and installers might resist changes that require more expensive components.

The Trend: Moving Towards Multi-Pole Breakers

Modern interpretations of the NEC and best practices increasingly favor the use of common trip breakers (multi-pole breakers) for MWBCs for the following reasons: Enhanced Safety

Common trip breakers provide a more reliable guarantee of disconnecting all ungrounded conductors in an overcurrent situation.

Reduced Risk of Backfeeding

Backfeeding occurs when one leg of a circuit is de-energized, but another is still energized, potentially causing a dangerous voltage on the disconnected circuit. Multi-pole breakers eliminate this possibility.

Clearer Code Interpretation

Eliminating the ambiguity around handle ties simplifies code enforcement and reduces potential for improper installations.

In Summary:

While the NEC technically allows handle ties on single-pole breakers for certain MWBCs, this practice is becoming less common and less favored due to safety concerns and the potential for misapplication. Using multi-pole (common trip) breakers for all MWBCs is the safest and most reliable method and is becoming the de facto standard. Your understanding of the potential issues is accurate. Flag for review