Singapore is not the US, we cannot afford dedicated aircraft for
every tasking, as such we have to get aircraft which can fulfill a
number of roles reasonably well. In fact the US is finding itself
in a similar situation, though much less so, hence the scrapping of
the EF-111 and letting the Prowler take over USAF EW duties. The
trend today is using multirole fighters which can take on air
superiority, deep strike, CAS, BAI and a whole lot o other roles.
This is evident worldwide, not just restricted to Singapore
As for the US$2 million A-4, is that in 1970s dollars? 
And besides, CAS is not the A-4s only job. The A-4 in the RSAF has
more roles than that.