I have tested out the videocall user experience extensively. My personal estimate (not the official MTN view) is that videocalls are very useful for specific contexts but will not easily replace voice for normal daily telephonic communication. It is simply a lot more work. The benefits are when showing something and for more personal conversations (eg with family whilst away from home).
To get back to your question of why you would want to stay on MTN: Because you are on a Procall 300 package, from Sunday 26, if you upgrade to a USIM you not only get a 128 kilobyte SIM card with extra address book fields, but enjoy a three month promotional period which includes a videomailbox, 300 megabytes of free downloads and 75 minutes of free videocalls. We've also prepurchase some of the video content on MTNLoaded so people can try that out!
I therefore hope you will take advantage at least for the three months to try out the data and videocall bundle and video content! It's a dynamic industry so we can talk again in October.