You're right. The best players do use an alternating fingering for that trill. Why? Because it changes the vibrato... It's a little more expressive, since its easier to shape alternating finger trills. Also, It makes the bottom note of the trill a little bit louder. It's not any easier, although it's more stable than a two finger trill. I agree with toca though, that you should use the five on the lower note. Although, that won't produce a very fast trill. I still hold to my 3-5 fingering. Although it'll be a little harder to hold the lower note. Would you post a picture of the passage? that would make it easier for us to give advice.