Gay people got really angry at Elton for doing this & even protested so I think if they didn't even announce it would be way more shocking for both sides...
Em took a lotta heat by his rivals & not friendly hip-hop fans for doing this & Elton took a lotta heat by gay people who hated the idea... probably it was a must to be announced earlier.
The performance was definitely a must 'cuz the anti homophobic wave might have actually had some backlash on EM since back then MTV & music TV's in general were the bar so if the pressure kept going TV stations might have backed off & that would have been bad considering how MTV had vital role in EM's career back then.
It actually served its purpose just fine... EM came clear out of it saying ''WHAT, I'm not homophobic!''

On another note... get ready to change that avi come Tuesday night!
