For literally the past century, the majority of astrologers have been using physical birth times to calculate horoscopes. Through no fault of their own, this is all they have known and been taught. Knowledge of spiritual birth (Incarnation) times has never been presented to them, except through the Edgar Cayce readings—and even there it seemed like an archaic and ambiguous subject because no explicit means was ever revealed on what it was, when it occurred, and how to arrive at (or calculate) it. So astrologers have done, and still do, the very best they can with what they know.
Also, astrology is so extraordinarily revealing that glimpses of accurate information invariably come through even when the birth time is inaccurate. Add to that the fact that there are some exceptionally gifted astrologers in the field who are fantastic at what they do. Some have an uncanny capability to intuitively ‘pick up on’ astrological energies and communicate valuable information no matter how inaccurate a horoscope is. One reason is that, in regards to a natal chart, the positions of the planets in signs and the aspects they make to other planets move very slowly and so pretty much stay the same for a specific 24-hour day (with the exception of the fastest-moving indicators: the house cusps, including the major angles, and the Moon). This explains why astrology has survived for so long and is still in current use.
That being said, what changes and impacts a horoscope and person’s life considerably is the rising or personality sign (Ascendant). It directly affects and shifts all remaining inner house cusps, planets in houses, aspects (including aspects to the major angles), and forecasts. All of these elements depend on the correct Incarnation time and horoscope.
While no physical birth time has ever produced a 100% functional horoscope, there are times when it at least has the correct Ascendant sign—just in the wrong degree (usually at a later degree from where it should be). Having the Ascendant even one degree off (or four minutes) dramatically impacts the effectiveness and accuracy of forecasts. A one-degree change may also change the Midheaven sign, especially if it is at a very early degree of a particular sign.
A one-degree difference on a zodiac sign is not the only thing that matters. The accuracy required of Incarnation times cannot even afford differences in minutes! For instance, a ONE-MINUTE change in time equates to a 15-minute difference on the Ascendant. A four-minute change in time equates to a 60-minute (one-degree) difference on the Ascendant. Hence, Incarnation times and calculations for accuracy must be measured in degrees: minutes: seconds: and milliseconds (where possible). That’s the level of precision and exactitude needed in order to obtain a 100% functional horoscope.
So what often transpires among mainstream astrologers is an imperfect but moderately workable system of astrology. One that has the advantage of good astrologers who can extract and render helpful astrological information—but also one in which doubts creep in on the receiving end of consultations (i.e., from clients) because there is almost always a blend of half-accurate, half-inaccurate information by default of using physical birth time horoscopes.
This does not even begin to cover other areas in the field of astrology that tend to get a bit more messy—such as non-agreement and use of various house systems, debates between Tropical vs Sidereal astrology, use of asteroids and other space objects in horoscope interpretations, etc.
Again, all of these matters can be exponentially improved to the point where there are zero mismatches, errors and inconsistencies if only the correct Incarnation times and horoscopes are calculated and verified.