I was on the Colorado Trail portion of the Continental Divide trail during June(2010).
I used a Western Mountaineering Ultralight throughout the Mexico to Canada trek including the stretch where the two trails begin to coincide northbound; Elk Creek Drainage.
I mostly used the WM ultra bag as a quilt all through Colorado and that is what prompted me to switch to a quilt (MLD Spirit quilt 30) for the Appalachian Trail this year.
The conditions i found along the Colorado Trail in early june were sunny cool mornings from freezing to temps in the 40's to thunderstormy afternoons which were in the 50's to low 60's.
I used a Gossamer gear One tent during that time and keeping my down bag dry was no problem as i always had sunshine to air out and dry my bag everyday at some point.
From just North of hunchback peak and the southern confluence of the CT and CDT, all the way to Twin Lakes and Mount Elbert, where the trails diverge, I used my WM bag as a quilt without problems.
Here is a picture from Hunchback peak at the southern end of the CT where it meets with the CDT in early June.
After reading Eric's post below i wanted to add.. I used the WM bag as a quilt and sometimes wore clothes underneath to bolster the warmth.
A sleeping bag, used as a quilt, works, but has dis-advantages.
There is too much girth, so that a lot of extra material is just laying to the sides and not contributing to insulation, or worse: it opens up to drafts when you move.
The quilt, by design, "cups" your body even when you roll over so that warmth is retained.
However, i would never have figured out how to use the quilt properly if i had not used my bag as a quilt first.
The other advantage of a propely sized quilt is that you can add layers of clothing without compressing the fill of the quilt.
A bag has a definite inside circumference that can be exceeded.
A quilt lays on top of you, and your clothes, in an arc.
With my WM bag if i added layers of clothes it worked to a point until the clothes began to compress the fill of the bag from within and actually decreased insulation, and restricted blood flow to extremeties.
Personally, i would recommend making slow steps.
If your present sleeping bag is adequate to freezing, or a bit below with some clothing, go with that.
Use it as a quilt and see if you like it.
Then if you find it favorable, buy or make a quilt, if thats what you want.
My 2 cents