In almost any typical production, professionals, students and hobbyists alike, rendering in multiple passes is VERY common to achieve the look and control over a given shot.
Esspecialy when it comes to particle rendering.
It is not just common, but often MANDATORY for 2 reasons:
1. In post having all the elements in a shot as a separate pass allows for the ultimate control over a multitude of things such as, time of day, light and color, matte control etc. and much more.
2. In the case of Maya 8.5 and earlier specifically, you had no choice. However, because hardware rendering uses the HARDWARE of your graphics pipeline, those renders are SUPER fast!
In most cases hardware particle rendering is near real time, around a second per frame depending on particle density, and multi-pass count.
Also, with Maya you have the luxury of assigning your hardware particles (and anything else in fact) to its' own render-layer.
This means, you can set per-render-layer override flags so that some of your layers render with MR, other layers render with the Hardware renderer and others with the software renderer.
All by simply right-clicking on the layer and telling it to use a given renderer overide
This may sound daunting to anyone using Maya, but this video made by Alias after Maya 7 added these features explain the render-layers feature very simply:
Just let it load
https://downloads.alias.com/mkt/gmk_m...der_layers.swf
Also, good job t1ck135 for pointing that out about the new feature in the new MR core as a LOT of Maya users STILL do not know that this is possible now
From the Maya 2008 "NEW FEATURES" list:
mental ray 3.6 Core
Maya 2008 uses the latest mental ray 3.6 core.....
...Additionally, particle types previously supported only in the Maya hardware renderer can now be rendered in mental ray, eliminating the need to combine outputs from multiple renderers. (i.e composited in post)
Originally posted by t1ck135
.....unless you instanced geometry to each particle....
Dont quote me on that last bit though 
Too bad! I quoted you! hehe...
You are correct again
And not only does the instanced geometry render in software, it also renders in Mental Ray in 8.5 as well.
...not sure about v8 or earlier tho.
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com