Does a slender pillar grant just bonuses or does it also grant other benefits?

Question

Miscellaneous Features on Slender Pillar says

These pillars are only a foot or two across, so they don’t occupy a whole square. Place a dot in the center of each square that has a slender pillar in it, and don’t worry about exactly how much space it takes up. A creature standing in the same square as a slender pillar gains a +2 cover bonus to Armor Class and a +1 cover bonus on Reflex saves (these bonuses don’t stack with cover bonuses from other sources). The presence of a slender pillar does not otherwise affect a creature’s fighting space, because it’s assumed that the creature is using the pillar to its advantage when it can. A typical slender pillar has AC 4, hardness 8, and 250 hit points. (Dungeon Master's Guide 64)

Does a creature sharing a square with and using to its advantage a slender pillar only gain the listed bonuses or does the creature also gain the additional benefits of cover? Specifically, is such a creature immune to attacks of opportunity because a foe "can’t execute an attack of opportunity against [a creature] with cover relative to [the foe]" (PH 151)?

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Background: While looking for inexpensive ways for monsters to protect themselves from a particularly vicious tripping PC (that is, the PC really trips foes; that PC isn't, like, on drugs), I saw the 1st-level Drd spell climbing tree [conj] (Complete Mage 99) which, for 1 min./level in an adjacent square of soft terrain (e.g. dirt, sand, soil), creates an up to 10 ft. tall/caster level tree (hardness 5, hp 120) that can be climbed with a Climb skill check (DC 5) but can’t be animated nor be used in conjunction with the 5th-level Drd spell tree stride [conj] (PH 296). The climbing tree is otherwise like a slender pillar (DMG 64). (For this reason spells seemed a more appropriate tag than, for example, dungeon-design or terrain.)

My hope is that the spell climbing tree is cover enough to prevent attacks of opportunity, but the phrasing of the slender pillar rules is imprecise enough that I can't be certain.

Official rules preferred, but, in their absence, house rules are acceptable.

Answer 1

The presence of a slender pillar does not otherwise affect a creature’s fighting space, because it’s assumed that the creature is using the pillar to its advantage when it can.

This is the critical part IMO. Based on this, you can't claim cover in addition to the other bonuses already given (as that would be the pillar having an affect upon the fighting space). The thin pillar provides a cut-price version of cover, lacking things like AoO protection.

I would say, however, that there's a great case for you to propose that the presence of such a pillar makes trip attacks difficult or impossible; either as a house rule, or - if the trip attacks in question rely on sweeping the enemy's legs with a wide arcing attack rather than grabbing a body part or battering them flat by sheer force (e.g. this negates the ever-popular Spiked Chain or a sweeping haft strike with a spear, but not a Trip made by snagging a limb more directly with a hooked polearm or whip) - by direct GM ruling as an obvious consequence of physical reality (the pillar is in the way, similar to the inability to swing a 7-foot greatsword in a 5-foot corridor).

Answer 2

Unfortunately not.

Slender pillars grant cover bonuses, but they can't grant the cover condition to persons inside their square. The important part here is:

The presence of a slender pillar does not otherwise affect a creature’s fighting space.

which indicates that only the benefits listed are granted, and the normal cover rules (or other terrain rules) don't apply.

However!

The text also indicates that the lack of the normal bonuses and options one would get from fighting adjacent to a 1-foot diameter pillar is

because it’s assumed that the creature is using the pillar to its advantage when it can.

So if one manages to deliberately avoid using the pillar to one's advantage, then one's GM might allow the normal benefits to apply. Of course, benefiting from this would be using the pillar to one's advantage, so the situation is rather paradoxical, but there is at least some hope for your plan, particularly since you are the GM.