Dtrace essentially allows two things. The first is exploring a live system, similar to being in a debugger but without its overhead. The other is monitoring; it's possible to notice that something is wrong earlier and with less overhead than monitoring logs and testing services.

Both aren't strictly necessary/much used in practice for various reasons, but depending on what you're doing, it's very handy to have. In a way similar to ZFS vs Stratis: the first ready to go, well-made, convenient, while the second is an absurd mess made by people who don't understand operations and think they know better from their development machine.

On Solaris... In my opinion, the real problem is less about Slowlaris and more about having chosen to target the élite for so long instead of targeting students, who are the future technicians, managers, etc.

GNU/Linux succeeded IMVHO because it was aimed at this audience, which is much broader, with many who weren't interested, but it's also the group that shapes 100% of every future generation of decision-makers and technicians. When SUN realized this, first with SXDE/CE and then OpenIndiana, it was, yes, damn late.