It's very room dependent because the M2's port is tuned around 27 hz. There's two settings in the Crowns for bass extension, one that aims for extended bass by boosting about 4 dB centered around 21 hz or so. In my room I don't use that low-end boost and still have to cut the sub 30 hz by a considerable amount due to excessive room gain, but in larger rooms with less room gain there will be a steep cut-off below the port tuning and the boost will only result in more flapping of the woofer.
So "needing subs" is relative. In any case, the 2216 woofer is protected by extremely stiff suspension and will likely explode before bottoming out, making it perfectly suitable for PA and loudness, but not so perfect for hifi and accuracy due to modulation because of quickly stiffening suspension with increasing excursion.
If you decide that it's better to go more complicated with more ways, then off-loading the main effort to subs will allow for better suited mid-range drivers - and likely positive gains in sound quality because of it.
The differential drive motor also has a self-braking mechanism, so it's very unlikely to be driven too far mechnically even if suspension had greater compliance.
I also suspect the risk of modulation distortion ever becoming audible is minimal in a domestic use case (smallish room with listening distances under 5M) for three reasons:
1) The excursion of the cone is actually decreasing as it gets closer to the port tuning frequency
2) Output would likely be very (dangerously?) loud before it would become an issue
3) The inner portion of the cone decouples from the outer portion attached to the surround as frequency increases, so midrange frequencies may not be as modulated by suspension stiffness due to low-frequency excursions.
All bets are off if the cone excursions aren't controlled below the port tuning frequency.
I've tried to hear an improvement in mid-range quality when loudly playing deep bass-heavy music with and without subs, and I can't say I've ever been able to hear a clear improvement. When I've done the same exercise with the 305p the improvement is revelatory.
Edit: Despite the commentary above, I still get peace of mind from running subs
Edit: typo (breaking to braking)... I hope the 2216 isn't self-breaking!