Most major Japanese dictionaries have an entry for つかえる in addition to an entry for つかう.  See for example Meikyo's entry for it:
つか・える【使える・遣える】ツカヘル
1) [自他下一]〔「使う」の可能形〕使うことができる。「武蔵は二刀流が―」「会議室は使用中で―・えない」「修理すればまだ―・えます」
2) [自下一]〔[1]から〕
 a) 有能で役に立つ。「もっと―人がほしい」
 b) 刀剣などの術にすぐれている。「お主、なかなか―な」
==
Sense 1 is the potential form.  Sense 2 is the additional meaning "to be useful (in a certain position)", "to be talented", etc.  GG5 (the gold standard of Japanese-English dictionaries) also has a separate entry for 使える that matches ours:
つかえる【使える】 (tsukaeru)
 〔利用可能である〕 be usable; be of use; be accessible; be available; be fit for use [service]; 〔役に立つ〕 be useful [helpful, of help, good] 《for…》.

@@ -15,0 +15,2 @@
+<gloss>to be usable</gloss>
+<gloss>to be serviceable</gloss>
@@ -16,1 +18,0 @@
-<gloss>to be serviceable</gloss>

http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1W�Ȥ�_v5u

From even reading the example sentences listed, it becomes clear that this entry is merely a conjugated "capable" or "potential" form of the verb つかう.  It does not truly mean "being useful."  Many things "can" be "used", but that doesn't make them useful. 

I have found no uses of it in its own potential form (つかえられる) on JMdict  but I have found about 7 thousand hits on Google.  Using it this way seems redundant, as if one is saying "___ can be capable of being used."  Perhaps it is being confused with useful?  If not, why so few usages of it in its own potential form?

Since this appears to be a conjugated form of a different verb, I believe it should be deleted so as to not confuse people with more appropriate words such as やくにたつ/やくだつ.  Of course, I am not an expert so please take my suggestion in that light.

meikyo

@@ -6,0 +6,3 @@
+</k_ele>
+<k_ele>
+<keb>遣える</keb>