OK, that may be a bit harsh. But the danger is that a translation that is out-of-date or badly done will turn you off the book. Many classic books whose translations are now beyond copyright are available for free. But these translations are, generally speaking, poor. To really appreciate these books, find a translation that is up-to-date and that suits your reading style.
When you also have the original text, whenever there is a more interesting or obscure paragraph you can look to see what was really said, possibly with the help of a dictionary.
Even when the translation is good, the translator cannot stop at each sentence and explain why certain English words have been chosen, which may be the closest to what was said, or they may be not, but the translator has thought that the chosen translation is easier to understand for an average reader.
The older translations (and perhaps the future translations, taking into account the current trends) also avoided to translate whatever words were considered offensive when the translation was done.
Knowing the letters is enough to allow the use of a dictionary to find most words, i.e. most nouns. Searching for verbs in a dictionary can be more difficult without knowing the grammar, as it may not be obvious which is the dictionary form that corresponds to a verbal form in the text.
I have read many Greek and Latin bilingual books and I have always found the original text to be of great value. The English text is very useful for reading quickly in order to have a general idea about the content of the original text and for searching quickly things in which you are interested.
Whenever you want to know anything certain about the content of the original text, the only way is to look at the original language. It does not matter if the original text looks like the original inscriptions. The original text may be shown in a one-to-one transliteration into Latin letters, without losing any information.
On the other hand, I have never seen any reliable English translation, i.e. any translation where after seeing twice the same English word in the translation you may conclude that the Greek author used the same word in both cases or that the author meant the same thing in both places.
Moreover, almost all translations that I have seen contain some anachronisms, i.e. modern words which do not really have any exact correspondent in the ancient languages, so when looking at the original you can see that the Greek or Latin words actually meant something else. Because of this, I have seen papers in which wrong conclusions were affirmed about what some ancient authors have said, due to the fact that some translations were accepted as being true literally, without checking the original words.