@albertchui,
This is not really a CDSEO question, so I will let you know what I think...
In short, if your country / language focus dataset is not offered by those keyword tools or you have a lot of spare time, then you don't need them... but read on before deciding not to use them...
Firstly, there is a difference in the focus of these tools - namely:-
1) Link-assistant is a great all-around seo-tool which honestly, I would be lost without - their Ranktracker tool, can more quickly help "suggest keywords" - but note that it can be slow to use to pull "# of searches data (used for calculating KEI)
2) Keyworddiscovery and Wordtracker (and SEMRush) are keyword discovery and keyword data tools - they pull lists of keywords and KEI data very, very fast.
Since you are asking about link-assistant - I will comment on that first for keyword research.
This is a great tool for finding a list of suggested keywords relating to your niche - especially for non-English websites (i.e. using "suggest" in Ranktracker or scrapping from top sites using SEOSpyglass).
There is however one downside of link-assistant when it comes to pulling keyword data... namely that these days, it is hard to easily pull the "# of searches" data using it (basically Google locks up very fast these days when pulling "# of searches" data - really you can only pull 1300 results when logged into a Google Webmaster account).
So why is this an issue? Well, as you know, once you have collected your great list of keyword suggestions, you need some way to compare / rank them for value - the most common method is KEI (and to calculate KEI you need to know the "# searches" per month data!).
So link-assistant is slow and at times pull-your-hair out annoyingly slow for pulling "# of searches" (per month) KEI data - this is the point you should know.
To answer do you need Wordtracker, Keyworddiscovery or SEMRush, depends on a few things:-
A) WHICH COUNTRY/LANGUAGE MARKETS IS YOUR WEBSITE TARGETING / ARE YOU WANTING TO FIND KEYWORDS AND KEYWORD DATA FOR?
A1) Wordtracker pulls data from USA and UK and is based on dogpile, metacrawler and Google data (I understand that only for Google results database only, can you narrow it down to English by a specific region i.e. USA, UK, Australia, NZ, India and South Africa - for data from other sources you get USA data only - but you will need to check that with wordtracker staff to be 100% sure).
A2) Keyworddiscovery - data is available for several countries (and therefore languages) - see this link for more info
http://www.keyworddiscovery.com/kd-manual-database-regional.html
A3) SEMRush - data is available for several countries - see this page and click on the country drop down next to the search box to see the list of regional databases
http://www.semrush.com/info/zappos.com
NOTE: you will notice that for some country/language markets these three tools do not have any data and therefore can't be used for anything more than keyword suggestions i.e. can't be used for KEI (e.g. Asian markets) - link-assistant however can pull data for any country/language combination (or be it slow sometimes)!
B) HOW MUCH SPARE TIME DO YOU HAVE?
As discussed above you can pull "# searches" data manually very slowly or using link-assistant a little bit faster (a lot faster for "competition" data however, if you use rotating proxies) - the advantage of Wordtracker, Keyworddiscovery and SEMRush therefore, is that you can get "# of searches" and KEI data very very fast and also large keyword lists (the downside is that they are paid solutions and don't have keyword data for some countries / language matches)!
I hope comments help - cheers, Asiaplay
EDIT: Changed Ranktracker "# Searches" to 1300 per time (about 3 minutes to pull that data)... you can of course add more g-mail & google adwords accounts if you have multiple IP addresses you can use (to have multiple Adwords accounts) so you can pull more data than 1300 per day (but it still means there are limits in the amount of data that can be pulled and the time it takes to pull the data).