For some reason, it seems as if everyone's grandfather at one point in time recommended to use the right tools for the job. That's practical wisdom from my point of view, one that is certainly an issue for agile development.
One of the primary messages, I hope, of the Agile Scaling Model (ASM) is that context counts. Although the focus of the ASM is on describing a contextual framework for tailoring your process to meet the needs of the situation that you find yourself in, it's also applicable to your tooling selection. For example, the tool choices of a co-located team will be much different than that of a geographically distributed team. A co-located team will likely use a whiteboard or paper for their agile modeling efforts, whereas distributed team members may need to capture their diagrams using a more sophisticated tool such as Rational Requirements Composer (RRC) so that their work can be shared electronically. Having said that, RRC would be overkill for a co-located team (unless they had regulatory compliance issues). Different teams, different situations, therefore different tooling choices.
One of the concerns that I run into from customers is that some of our legacy products don't support agile very well. Once again, it's a matter of context because many of our legacy products reflect the realities faced by more traditional teams. The challenge occurs when you try to take a legacy product which is well suited for traditional development, such as Rational ClearCase, and try to apply it on agile projects. Although ClearCase makes sense in certain scaling situations, particularly very large teams that are geographically distributed, you'd be better advised to use something like Rational Team Concert (RTC) for configuration management on most agile teams (note that RTC does far more than just SCM).
So, if you're taking an agile approach you should consider Rational tools such as RTC, RRC, Rational BuildForge, Rational AppScan, and others which support agile development. Granted, some you would only use at scale -- for example Buildforge is a good option in really complex environments, but if you don't face that complexity then you'll likely find that RTC's build engine is sufficient. Similarly, if you're taking a traditional approach to development then you'll likely consider products such as ClearCase, Appscan, RTC, and Rational Software Architect (RSA) instead. Different situations, different tooling choices.
What's even more confusing is that some products support a range of process paradigms. For example, RTC supports agile, lean, iterative, and traditional approaches to development. The same can be said of Appscan and several other products. Notice how I listed RTC and Appscan for both agile and traditional development above.
So, if anyone tells you that Rational tools don't support agile development don't believe them. Ask them which tools that they're talking about, and ask them if they're aware of the Rational products that do support agile development. Context counts.