The Undertaker should keep his streak. Like others said, he can put over other talent any other time of the year. Hell, he put over Jeff Hardy a month or two ago. It's part of Taker's backstory, and should remain as such. Plus think of the historical part. If Taker loses his streak, then the streak pretty much loses much of its prestige. Yeah, it's still pretty awesome if he is whatever-1 at Wrestlemania, but whatever-0 sounds much better. It'll be a record for the voice-over guy to go on about over Taker's HoF video when it comes around in however many years after Taker's retirement.
And you gotta admit, Taker steps up his game exponentially at Wrestlemania. Many others do too, like HBK for example, but Taker is a stand-out.
Which leads on to that I think HBK vs. Taker would be one hell of a match. HBK doesn't need to break the streak. He's big enough. He's already the man who retired Ric Flair. But, he could sure have a fine attempt at it, and these two could and probably should, if paired together, put on the match of the night, and probably contend for Match of the Year. (Unless something unbelievably awesome happens somewhere else).