Fleece is relatively light, inexpensive, and works well in a wet climate. It does not have the loft to weight ratio that polyester and down garments provide and doesn't compress as well. I don't think of it as extreme cold weather gear, but for moderate cool, wet weather-- great stuff on a rainy 45F day. Rain parkas and fleece are paired staples in the Pacific Northwest.
Like so many "lower performance" garments, fleece is great for day hikes and short trips. Many great clothing options are good for more single-purpose pursuits. I have a Marmot DriClime windshirt that is a wonderful garment for all kinds of outdoor use, but it doesn't fit into an ultralight multi-day scheme as it weighs 16oz and is replaced by a more vesatile Montane windshirt and a Patagonia MicroPuff vest. For a day hike or around town walks, it is one of my favorite peices of gear. Likewise softshell garments-- great for a day of downhill skiing, but too heavy for the utility provided in a multi-day ultralight trip.
I have 200 weight fleece vests and sweaters, several 100 weight fleece tops, some fleece bottoms in 100 and 200 weight, and a great Power Stretch top from Mountain Hardwear. I think of the 100 weight and Power Stretch as a heavy base layer option rather than a light insulation layer. The stuff wicks well and is good against the skin and perfect for sleeping in.
The high loft layers fit into the high performance demands of ultralight hiking where every gram is scrutinized. When you equip for thru-hiking and higher altitudes, I think the high loft garments make sense on a warmth to weight basis. For less demanding trips, cool and wet conditions, and all-round utility, I think fleece works just fine.