The advantage of the decimals is that it allows different degrees of precision, depending on what is needed. So anything from ancient Sumer to the Reformation is Level 1, but I can also say that the Renaissance is level 1.8.
Another advantage is that if you see a tech level at 0.9, 1.9, or 2.9, you know that it is a society about ready to move into the next stage, and likely to be more in flux than a society at 1.5, 2.5, or 3.5. This is because the levels have been chosen to map to broad paradigms (agricultural vs industrial vs cybernetic). The stages, besides indicating what kind of technology to expect, also indicate how far into the broad paradigm society has evolved, so at 2.0 or 2.1, people are just getting used to "industrial" ideas such as urban living and factory work, while by 2.6 or 2.7, people are taking these ideas for granted.
By 2.8 or 2.9, people are beginning to see the need for a new paradigm. (That would correspond to the period about 1960-1995.)
But the most useful aspect might be the ability to talk about the future. The GURPS tech levels for the future seemed kind of vague, while my system divides the future into three major eras: the cybernetic era, where technologies that we can see as engineering problems are developed, such as artificial intelligence and interplanetary space travel; the interstellar era, where "magic" technologies, i.e., those like FTL travel and force shields which have no solid basis in 20th century science, are developed; and the advanced era, where technology is so advanced that we at the beginning of the 21st century can hardly imagine what these would be like.
The hard part, though, is that we can't really predict in what order things will be invented. Maybe somebody will invent a workable interstellar drive in 2009. Or maybe it won't happen for a thousand years, or even never. It is very speculative.