The argument against scheduling the 'do' is that it decreases the fun factor and makes it an obligation. This article "Many Couples Plan Ahead To Get It On" by ABC News features a discussion on that. I usually ask couples how often they were bonding sexually without any conscious scheduling. The response is universally "we do it when we do it" - in other words they often experienced infrequent sex which is okay if both partners are in agreement. Researchers state a marriage experiencing only 10 encounters in a year is a sex-starved marriage. If this is your case, talk to a local licensed therapist or check out http://www.smartmarriages.com/ for helpful resources.
Most couples struggle with the old age 'lack of time' problem, we should just be honest and say we haven't made the bedroom life a priority. We let everything else take first place in the marriage but relegate the important portion of sexual bonding to the back burner. I love what Michele Wiener Davis (a couple's therapist) tells couples - "Think of Nike and Just Do It"!
If your relationship is close to this category here are a few of my favorite resources/books on the topic.
Crazy Good Sex: Putting to Bed the Myths Men Have about Sex by Les Parrott III
The Sex-Starved Marriage: Boosting Your Marriage Libido: A Couple's Guide by Michele Weiner Davis
http://www.themarriagebed.com/ is a helpful site for faith-based couples
I will post a follow-up on this one since I have to go into session in a few minutes.