Yep, chicken and egg scenario. If more people would buy Windows Phones then developers would make more apps. I feel that the WP OS is worth not having a few niche apps.
Yahoo is quite close with Microsoft, they could make a WP app in one week, a good Y!Msgr app for Windows Phone was "QuickYM", but it received a copyright infringement from Yahoo and they forced him to exclude the app in US and Canada.
Yeah, I also use IM+. I was using it on Windows 8 for a while as well, but got sick of it and moved back to Trillian on my PC. The app on the desktop kept wanting to go to sleep mid-conversation, and I got tired of dealing with it.
Really, Trillian is the IM client that we need on Windows Phone. It's like IM+ but better.