First, there is the matter of code. The 2009 version of the IRC spells it out. See comment #1 in this discussion:
http://www.greenbuildingadvisor.com/...venting-inside
While some towns may still be governed by 2006 code, venting inside is a bad idea, for building science reasons. An electric condensing dryer can be vented inside, because the moisture is removed from the air. An unvented conventional electric dryer still dumps too much moisture into the air, even when it is felt that moisture is needed.
A house that has very dry air inside in winter is a leaky house. Take today, for example. The local weather has the dew point at 13 F. Outside air that leaks into the house through all the myriad cracks and holes in the outer shell of the house and gets warmed up to 68 F then has a relative humidity of less than 3%. That's really dry air, although it is mixed with air inside that has been humidified somewhat by human activity (breathing, cooking, bathing, plants, etc.). Air that leaks into the house is displacing other air already inside, which leaks out through other cracks and holes. That air passes through the walls and ceiling below the attic, where it contacts surfaces that may be below the dew point of the inside air. The wood absorbs the moisture, which can lead to rot and mold.
Of course a really leaky house may simply leak so much that the wall cavities are continually dried enough so that no moisture accumulation occurs. One way to address the symptom (dry air) is to tighten the building shell to reduce air infiltration. In a very tight house, built to be exceedingly tight, mechanical or passive (but controlled) ventilation must be provided, both to provide fresh air to the occupants and to avoid excessive inside humidity, even in winter. Human occupancy does humidify the air. In an older house, not built with tightness in mind, it would take extreme measures to tighten up the shell to the point where deliberate ventilation would be required. It's simply too difficult to get at all the leaks post-construction. Air sealing is still worth doing, however, and is the first thing to do to reduce heating cost (the "low hanging fruit").
Otherwise, addressing dryness by humidification can be overdone. That added moisture has to go somewhere, ultimately, and through the walls isn't a good idea.