It certainly does. Using a keyword makes a simple alphabet substitution comparatively much stronger in one easy step, but only if it's not something easily guessed.
And besides, any substitution cypher is still subject to a letter frequency analysis. Provided the message is long enough for average frequencies to apply, you can disguise the base message all you want, but in the end it'll still always be subject to the same letter frequencies in English.
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And besides, any substitution cypher is still subject to a letter frequency analysis. Provided the message is long enough for average frequencies to apply, you can disguise the base message all you want, but in the end it'll still always be subject to the same letter frequencies in English.