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.