There are a few advantages for you and your customer when you release frequently (monthly or more). Here's one possible train of thought-
--- thinking like the customer---
Pro - Smaller changes are easier to handle
Instead of giving me one huge drop every six months or year, I get a smaller drop more frequently. I don't have to train users (and be trained) on a lot of changes that affect me, instead the delta is small and easy to manage. I can take a few changes in the software and focus on adapting to them. I have time to learn how to leverage this new functionality to my needs without splitting focus in as many directions. I can provide feedback to the software team based on what I learn instead of waiting for six months in the dark and wondering whether they will change that thing I complained about last release.
Con - the cost of change has a base price
But, it costs time and money to take on an upgrade regardless of how small or large. This minimal costs is never less than $X. This $X cost multiplied by the number of releases per year can cost more than the old single release approach cost me.
Pro - the customer chooses to take a new release
Ahh... but I don't have to take every release. Sometimes releases have changes in them that my group doesn't need or use. We can completely skip a release (or two). If I know what is in the release (and it's a smaller list to review now), I can very quickly decide to move along because there is nothing for me this time. End result, I only take releases when their value is worth investing in taking.
--- thinking like the software team---
Con - our customers are on many different versions of our software
Oh crap, the customer base is on 20 different versions of my software. How do I support all of this? Yeah, that kind of sucks. Is there anything you can do about it? Well, you have to have a good CM approach. And you have to make sure your team makes smart choices between when to fix/patch something vs. forcing the customer off of a version.
Pro - it forces you to provide value in each release
But wait, there is a way to help get people off of your older versions of software. If you are building something of value in each release, then customers are more likely to take the drop. Fix the things they don't like, add features they ask for, improve performance and next thing you know... they want to upgrade.
Con - you gotta stay on top of the market and customer needs
But this requires me to know what my customers want! That's hard! I guess I need them to be tied to the team somehow. They need a voice in prioritization. We need a way to get their desires onto the backlog. We have to mold and evolve our strategy to mirror the changing market.
Pro - you are in touch with the market and customer needs
HAH! We figured out how to do that. It was hard, and it costs a little more money, but we make soooo much more money than before. Customers are loyal and love us. They tell others about our stuff. We know what the market wants. People buy our stuff. In good economies we grow market share by beating our competitors. In bad economies we grow market share by outlasting our competitors. Case studies and leads are easy to find. We are spending a little more money, but we are making a lot more money because we can charge a premium for something we know the market wants.
Do you have a story to share to reinforce any of these points?