You believe that BRK's underlying businesses, like See's or 8% of Coke, would be cheaper if you bought those companies directly than if you bought BRK stock. However, that assumption is generally not true. By buying BRK (when it itself is undervalued), you are able to buy KO, G, etc. for below their individual market prices. It's also an ongoing mystery in finance circles why mutual funds often sell below the collective value of their stock holdings, but it happens. As recently as 12/31/97, BRK stock was undervalued. It's "investments per share" was $38,000 and the intrinsic value of its wholly-owned subsidiaries was about $20,000, but BRK's market price at the time was only $46,000. You could have bought it then and gotten all the stocks in Buffett's portfolio on the cheap.
In addition, if you buy BRK stock, you benefit from Buffett's ability to obtain businesses (whether in whole or in part) AT A DISCOUNT. Maybe you have the same ability as Buffett to know when a business is selling at a discount or not, but most people do not. For example, Buffett is buying GRN now because he believes he can purchase it at a discount to underlying value (he's paying only $20 bil. for $24 bil. in floate). However, the average investor does not have the ability to determine whether a potential acquisition is selling at a discount or not. Buffett has the patience to wait until a stock he likes get extremely undervalued.
It's true Buffett's reputation adds something to BRK's stock price. But, as Buffett and Graham have said, markets are efficient in the long run (but not necessarily in the short run), so even BRK stock will be correctly valued by the market over time.
I disagree with your assertion that buying BRK reduces diversification. Quite to the contrary, I don't think you could get more diversified than owning World Book, Dexter Shoes, Dairy Queen, etc. (all different types of businesses) at the same time, which owning BRK stock amounts to.