As it turns out, my initial post about Monks using spears was erroneous, and derived from campaign specific rules- I've corrected it in my initial post.
I strongly disagree. In a combat with multiple opponents, you need to cull their numbers ASAP so the sheer multiplicity of attacks doesn't overwhelm your party. That's where the str monk shines.
I did a comparative analysis with a 2 Monks facing 4 opponents as a microcosm of a combat in which the party is operating in a "target rich environment."
Assumptions:
1) Each Monk 40hp, using a Greatspear (2d6 Reach- AFAIK, the highest damage reach weapon in WotC's publications), with +6/+1 attacks and a Crossbow for ranged combat (1d10); Str Monk has +4 to hit/damage in melee, no Dex bonus, Dex Monk has +4to AC & ranged combat, no Str bonus.
2) Opponents, each 30HP, using a longsword, no ranged weapon, 1 attack each, and no relevant bonuses.
3) Each blow is an average blow with no crits.
4) Str Monk loses initiative to opponents. Dex Monk has initiative over opponents. If a combatant is reduced to zero, it can only do its last bit of damage if it has the initiative advantage.
5) Each Monk did a ranged attack as the foes closed before switching to his spear, each used his maximum number of AoOs, and each concentrated their normal strikes on a single opponent until dropping it.
6) The foes concentrate their blows on each Monk until he is dropped. Any blows in excess of what is needed to kill a Monk go to the Monk's allies.
7) No allies interfere on either side.
8) No flurry was used, just basic attacks.
Results:
In each case, the Monks are overwhelmed by their opponents.
The Str Monk drops his first opponent after 1 ranged combat round and 2 melee rounds, and is dropped in melee round 4 without dropping any other foes (he got no dying blow since he lost initiative). The foes remaining had a total of 82.3HP left, and had 1 blow to allocate to the Monk's allies in round 4. They had all of their potential blows left for round 5.
The Dex Monk drops his first opponent after 1 ranged combat round and 5 melee rounds, and is dropped in melee round 5 without dropping any other foes (he beat his foes initiative, so got a strike in this round). The foes remaining had a total of 79.5HP left, and had 1 blow to allocate to the Monk's allies in round 5.
Analysis:
While the Str Monk drops his first foe much earlier than the Dex monk, his lower AC means he is overwhelmed by their numbers more quickly- in fact, before he can make a further significant impact. The foes he faced are freed 1 round earlier and with more HP than those faced by the Dex monk. The Dex Monk thus was the better ally to have here, since he 1) did more damage in the overall combat, and 2) occupied his foes for a longer period of time than his Str monk counterpart.