2 things i think are not so good on this build.
No focus on cdr: Unless you have lots of runes and masteries, 15% from randuins alone isnt allot. Alistars only point is his skills, which cooldowns last longer then the battle (you stun once, bump once and your done). 40% CDR is really good.
Why attack Alistar: You mention 2 things to do when ignored.
Bashing a tower is fun for alister with his ult up, but your team cant risk that. So you end up alone at tower. Surviving both tower and the pot shots from enemy carries is only possible till late game/item set is complete. Half a year ago when he was OP this was possible (ult lasted 15 sec for example), but not anymore.
The other point, though better, doesnt work the way you mention either. Alister in front, walks towards enemy team. They either keep distance =>tower situation. Or they walk by you. Lets say they have amumu as tank (he's very good, to make difference obvious). If you dont bump him, your team gets locked down. Who do you bump then, their melee dps, their main carry? The mage? There is more in a team then just one dude. Lets say you do all perfect and bump their carry into your team. You're ignoring the fact that their other 4 players are already wacking at your team. You do your trick, busted all your skills with 1 result. Your team is now a blob of 5 instead of 4 people. So after the trick, you have a 5vs4 situation with a cow running around with big armour/undying....
Im not saying he's useless and better a tank than no tank, but others do better. Alister is built around skills, good teams wont focus him (or any tank), so a little less tankyness for more threat/utility is my suggestion. I think Frozen Armour or at least Glacial Shroud are spot on for Alister. Come to think of it, I build him very similar (whith more cdr is added), but I disagree on the way you say it works. You just get ignored.
Tried new built (in a pubstomp, so not that great of a reference). Wanted to go manamune, so I went Manamune, Boots of Swiftness (merc wasnt needed), glacial shroud, banshee, sheen. All give mana to add to manamune ability and cdr reduction en defenses. Worked out well. Need to try it in a serious game.