"Need."  Funny word, huh?  I don't know if I can answer your question, but my experience might shed some light.

I have tried several different ways to move air from my HD500.  I began with a Marshall AS50R acoustic amp - i figured the fuller range would sound great, and it's meant for acoustics, so the Vax acoustic sounds are beautiful.  It's closed back, so I still use this in church or a pit orchestra where I have to keep the volume down.  The rock tones left something to be desired, however.  I did a few gigs with a floor wedge Behringer monitor up on its side, and while it was plenty loud, the EQ seemed all wrong - too crispy bright.  Most gigs I played over the last year with the HD500 were through a Tech 21 Power Engine 60.  I was very happy with the tone I got from this setup - each time I played I was happier with the minor tweaks I made to the 500.  The issue with the PE was its directionality - if you're off axis, it really doesn't give the same sound.  Since I wasn't micing to a PA when playing in bars or outdoor gigs, I was ready to upgrade. 

Finally, I just got the DT25 last week, and gigged for the first time this weekend, outdoors, with a 6-piece band.  The warmer tubey tone really sounded - and FELT - good.  I feel it added to my performance and so did the band.  I stuck to the Blackface Double model for the gig, with a screamer, reverb, and delay, building off the HD>DT presets from the 2.0 HD update, but playing at home, all the amp sounds are fuller and the DT25 sounds more "real" and "alive" than the PE60 did.  I am very happy with my purchase, especially because I got it for $725 off eBay.

So, like everything in life, it's a trade-off.  Get the DT for that sparkly tube tone, or stick with a PA for flexibility and ease of using the acoustic models.  Personally, I'm happy with my choice. 

Cheers.