The ANSI/SQL-92 standard states that strings have to be padded to the same length when compared:
3) The comparison of two character strings is determined as fol- lows: a) If the length in characters of X is not equal to the length in characters of Y, then the shorter string is effectively replaced, for the purposes of comparison, with a copy of itself that has been extended to the length of the longer string by concatenation on the right of one or more pad char- acters, where the pad character is chosen based on CS. If CS has the NO PAD attribute, then the pad character is an implementation-dependent character different from any char- acter in the character set of X and Y that collates less than any string under CS. Otherwise, the pad character is a <space>. b) The result of the comparison of X and Y is given by the col- lating sequence CS. c) Depending on the collating sequence, two strings may com- pare as equal even if they are of different lengths or con- tain different sequences of characters. When the operations MAX, MIN, DISTINCT, references to a grouping column, and the UNION, EXCEPT, and INTERSECT operators refer to character strings, the specific value selected by these operations from a set of such equal values is implementation-dependent. Note: If the coercibility attribute of the comparison is Coercible, then the collating sequence used is the default de- fined for the character repertoire. See also other Syntax Rules in this Subclause, Subclause 10.4, "<character set specifi- cation>", and Subclause 11.28, "<character set definition>".
Other than the obvious "it isn't in the standard", what would be the reasons against trimming the ends of the strings instead of padding them to the same length?