I'd disagree: higher point buy makes surprisingly little difference. An 18/14/11 array is achievable with the default point buy, and for most classes, having a 20 primary is going to be the best to-hit+defenses there are. The extra hitpoints hardly matter as levels rise (though 1st level they're still slightly noticeable). Extra point-buy do help a little - particularly by making players more flexible in terms of feat prereq's - but there's no noticeable raw power difference IME. Also, MAD builds become more competitive (but these aren't generally better - it's rather that they're no longer generally weaker). An extra feat or two tend to help a lot more than extra point buy (in other news, humans are quite good, particularly at low levels).
Still, and particularly if players aren't into optimization, extra point buy points will raise power simply by raising primary ability scores (which matter most).
1. 18/14/11 is most often not the best you can get... especially if the 20 is not raising Ac.
2. Higher Stats make skills better which is useful in a small party.
3. versatility is gold in small parties. So beeing able to multiclass more freely (which an extra point in wisdom or charisma could do opens up some nice multiclass options and could make a powerswap very very helpful)
4. defender classes often profit from high strength and high constitution and can make good use of dexterity although it is most often not a even a tertiary. So every point in dexterity can make a big difference. (actually dexterity is so much better by default than int, that everyone should consider increasing it. (stealth + ranged basic + initiative can result in having to face much less minions when the fight begins.
5. In a 3 player party having no versality can make you dead very fast. Actually, every party member should be able to take some hits and every member should be able to heal, even if it is with the heal skill.
this results in: a fighter with higher point buy is a much more resilent and sticky defender:
what would be your choice of attributes?
strength: 20 if possible
consti: 15 at least for plate armor
dexterity: 14 (15 at paragon for HBO and shield spec or scale spec)
wisdom: 20 if possible
ok, most conservative:
18 strength
13 const
14 dex
16 Wisdom
achievable with a longtooth shifter
ao, raising point buy by 9 points would result in:
20 strength
14 const
14 dex
16 Wisdom
adding 11 points results in
20/15/14/16 or 18/15/14/18
this spread cries: WARPRIEST with fantastic stickiness and ability to heal. (powerswap with some cure spells)
beeing most generous you could easilymake a 35 Point buy and have seriously stronger PC´s which are not overpowered (as eamon pointed out) but much more resilent and versatile.