Why is old-man Shaq a good investment for us?
Well, when all is said & done, it all boils down to matching with L.A. because there's no other team in the West who will beat LA!
And as we've see in the Finals, the Lakers bigs killed us yet Bynum was not even 100%
And as we've also seen in the Cav's sweep of LA this past season, Shaq can somehow neutralize Bynum mainly because of Shaq's sheer size (height & bulk-wise.)  And Bynum was 100% in both games against Shaq.
Now Shaq may be a shell of his old self but his enormous size does bother Bynum unlike Perk who even if physical (let alone the best defending center in the NBA), is still shorter than Bynum & less bulkier than Shaq.  
And so, undersized Perk doesn't fully contain Bynum, as we just saw in the Finals - & Bynum was not even healthy!  Now how much more if Andrew's 80% next year?
Of course, we do need Perk against Dwight but our main goal should be matching-up against LA's bigs because they killed us even with Sheed.  
Hence, we really need Shaq for Bynum since KG's got a handful with Gasol.

"More Shaq positives"
But even against Orlando did Shaq contained Dwight in their games this past season as SIZE does matter!  And that's why the Cavs brought-in Shaq.
Hence, he will also help us against Dwight next year as Dwight keeps improving that he almost ate us up!
At this point in his career, Shaq would want nothing more than another crack at the championship, only our C's will provide that not unless LeBron goes to Miami, then Shaq will rejoin his old team & former teammates Wade & LeBron.  
But LBJ going to Miami is very unlikely as it's "Wade's house" & so, we're still the best team in the East come playoff time even if LBJ & Bosh goes to Chicago while the other top free agents goes to NY & NJ.  Indeed, Boston's still the best team in the East as long as Wade & LBJ don't get together. 
Bottom line: we're the best team for Shaq to join if he wants to win a ring again!
Thus, Shaq & Brad Miller (to lesser degrees) will surely shore us up. 

"Some other points worth mentioning"
Moreover, Shaq has GREAT respect for Bill Russell that he always refer to him as "Mr. Russell."   For sure, he wouldn't mind playing for us with Russell watching in the Garden.  
Furthermore, KG & Perk has GREAT respect for Shaq that he'll surely won't mind playing with these guys. 
I mean, Perk is very vocal about Shaq being his idol while KG, when asked in national TV who the real superman between Shaq & Dwight, point-blank replied: "Shaq" unlike Tim Duncan who tried to be more diplomatic by saying MJ so as to not get drawn into that debate.  
And that was even before the bad blood between Boston & Orlando!  In short, KG had no hidden agenda in choosing Shaq over Dwight, that's KG's simply respecting Shaq's accomplishments over the years.
And so, Shaq will be welcomed by open arms in Boston not only by the C's but also the fans as well. 

"In conclusion"
We may get nervous & upset this coming regular season as our "veteran team" [with Shaq] stumble once again just like this past year but come playoff time, we will go all the way!   
Last but not the least, maybe if we quickly get Shaq will he then call his pal LeBron to come to Boston which is now feasible with Pierce opting-out.   
Our team then will surely win more rings for years to come as Shaq, KG, & Ray still has a good 3-4 more years left, then LBJ, Perk, Rondo, BBD (+ future C's players) continues the winning tradition.