It is good to have Java plus some other commercial tool like BPM, SOA, Business Intelligence. Java cam be self-taught, Pega cannot be. It is your decision. Check my analytical multi job offer decision making at my blog.
Part of the reason that Pega can't be picked up easily is that it is proprietary and the cost to own it is very high: only large institutions like banks or tech-crazy lottery winners can afford to buy it. If you think that your organization will be committed to using Pega for a long time and if you think you will be committed to staying with your organization for a long time then this just might be the right thing for you. However, if you're like a lot of developers who get wanderlust for new things to learn every 3 to 5 years, then you're going to find yourself unhappily stuck in a niche with limited options to get out of. Unless you keep your Java (or other technologies) skills honed in your spare time or stay involved with other types of projects, a move to Pega should be considered a long-term or perhaps a career-limiting (at best) or career-terminating (at worst) commitment.